TL;DR
The way Arena places the extra (non-player) resources is completely broken, which gives rise to some of the ways in which the map can be unfair. This is due to the middle land having some kind of ability to swallow anything that should be placed at its centre, and force resources towards its edge (or the edge of the map, if there are back pockets behind bases, as in some 1v1 spawns of Arena), and commonly clump them in strangely artificial lines and grids.
Long essay version, with examples and discussion of what is wrong with the map
All of the main versions of Arena share their main code regarding how the map is put together with the standard Arena that comes with the game. So all versions share the same problem. The extra resources (gold and stone which do not 'belong' to players) are supposed to be placed a certain distance from all players' TCs, and randomly scattered across the entire area which is at least that distance.
You can see in the picture above that ALL of the extra resources have gone into that little pocket on the right. This is because the min_distance_to_players for the extra gold and stone is 40 (so 40 tiles from the TC). That means there's only the far right side and far left side of the Arena available to place these resources on, because those are the only areas that are at least 40 tiles from both TCs. Still, the game had the opportunity to put some of the resources on the right and some on the left, but it chose to put them all in a small clump on the right. Note that there would be a much larger area to put the resources in on the far left side, if the game wanted to, but it still clumped them all together. It won't ALWAYS do this, but Arena definitely exhibits a strong tendency to clump its resources together.
In 1v1, this usually isn't a problem when the resources spawn in the arena, because they'll usually be off to one side, in neutral ground. The requirement for the resources to be at least 40 tiles from both TCs means the resources generally can't appear directly between the bases. However, as we all know, sometimes in Arena you have an extensive back area, the far reaches of which can easily be 40+ tiles from the TCS, and quite often the extra resources get placed in these areas behind a player's base if it's possible. Again, the game will show a strong tendency to clump the extra resources together, so quite often multiple extra golds will appear in these pockets behind players' bases. Contrast this to a map like Arabia, where the extra golds and stones have the ability to scatter across the map properly (though sometimes they won't, but that's working as intended according to the parameters defined in the map script).
In TG, the problem emerges more clearly, first of all because in larger maps you don't get the same extensive pockets behind player bases so much, meaning that the extra golds have to go in the middle. The player bases are generally on all sides, and the map is large enough that the extra golds can be at least 40 tiles away from everyone's TC and still fairly close to the walls. In this situation, what you'll see is that the extra golds, if they appear at all (we'll come on to this in a moment) will never appear particularly far away from players' walls - certainly not towards the middle of the map - and they'll usually be clumped together in a line or grid formation that meshes with the player golds, with fairly regular spacing. Often, this places too many golds too close to some players (or one team). As AoC players, we're used to imbalanced maps and we deal with it. At least the golds are not behind the enemy base in TG. But the middle of the map will be curiously empty.
So, what causes this clumping of the extra resources and an emptying out of the middle on Arena? It's to do with the way the map is made. The base terrain on Arena, unlike most maps, is forest, and the player lands and middle area are cut out of the forest and grass terrain is laid down. On most other maps, it's the reverse, with the base terrain being grass and forest terrain replacing the grass. There are two commands for this. One land is created for each player by one command. The other command places a middle area of grass which must stay a distance of at least 18% of a map edge from each border. This land has a slightly different grass than the grass used for player lands, you may notice. It is this land which causes the problem with the extra resources. The game tries almost as hard as it can to force the extra golds and stones to appear as far away from the middle of this land that creates the central 'arena' on Arena. This has an effect on where the extra golds and stones can appear. Quite often, they won't appear at all, because the game has no option but to place them near the middle area, because there is no area on the map that is at least 40 tiles from all TCs and yet isn't in the middle of the map (more precisely, in the middle of the land that creates the middle area). Take this screenshot:
The bright red rectangle is the only area on this map where the extra golds could appear, because everywhere else on the map is at least within 40 tiles of a TC. You will notice that there are no extra golds and stones in this area, because it's close to the middle, and Arena tries very hard not to put any extra resources in the middle of that land. It also tries to put resources into fairly evenly-spaced clumps in a line or grid formation, as I mentioned before. Here is an example of this occurring on another TG map where the player bases leave more area at least 40 tiles away from TCs, so that these formations of extra golds, stones and forage bushes appear outside of bases. Due to the clumping, all the resources have appeared next to three players on one team, nicely placed in one area which is mainly outside the walls of the pocket player on one team. If you divide the map into two halves, one for each team, literally every single one of the extra resources has been placed in one half of the map, with such regular spacing as to look almost deliberate. There was plenty of space on this map, where the player bases are mostly very far towards the edges, for a random scattering of resources, which is what should happen, to take place, but instead the map looks very artificial.
Here is an example of where I have modified Arena slightly to illustrate the problem better. I've made the middle land into dirt terrain, asked the map to place a lot more extra resources, and reduced the minimum distance to players. The result is massive load of extra mines appearing outside player walls in the conspicuous grid formations, but once again avoiding the middle of the map.
Now for the somewhat more technical part: how can we fix this, and get a random, broadly fair distribution of resources in the middle of the map?
It will presumably involve remaking the map without that curious middle land that seemingly has a black hole at its centre, or else adding some kind of property to it (and/or the resources intended to be placed upon it) Unfortunately, I'm not sure how this would be done without either introducing another problem or making the map stray too far from being Arena.
You could do it if you only had the middle area comprised of player lands instead of having a separate middle land which serves the dual purpose of creating the middle arena and giving players access to the middle. Without the middle land to join together the player lands and a command to then connect all the lands, you can't use other_zone_avoidance_distance to force player lands apart, because then the map would turn into Michi, as the zone avoidance distance would be filled in by the base terrain, trees. Without other_zone_avoidance_distance, player walls, given that you want them to be a reasonable size, can spawn too close together far too frequently, and will not uncommonly even go so far as to intersect.
Scripter suggested adding another land to the middle with a different terrain and forcing the extra resources to spawn on that terrain. But as soon as you add another land on top of the player lands and the one middle land, bad things start happening with the trees:
It gets worse. You obviously wouldn't want any extra gold to only appear in the middle. This isn't Gold Rush, you want your extra golds to scatter into the empty space properly. So, you have to make that land with the dirt terrain cover more of the arena - close to, in fact the same amount of space the original middle land that created all the problems occupied. So, suddenly we are back to square one, only with more than a single offending middle land, you can get this kind of thing going on. By making the dirt area bigger and asking the extra golds to only place on dirt, with a minimum distance to players enforced, the old problem has reared its head and the grid formation of resources with regular spacing, all clumped in one place where possible and avoidng the middle, has returned:
So, how do you solve a problem like Arena, and make those extra resources properly spawn into the right areas and scatter properly? I haven't a clue. But this curious behaviour created by the middle land is what has given rise to Arena's famed unfairness with back pocket resources in 1v1, and extra resources either spawning unfairly or not spawning at all in TG (due to the lack of back pockets or empty side areas).
Even if you just place extra resources for each player but try to make them spawn a greater distance away, they still encounter the same problems as long as you have that middle land and you ask them to spawn further away than right outside the walls (as extra resources should). But that middle land, as far as I can see, is the indispensable key to making Arena work as intended.
The way Arena places the extra (non-player) resources is completely broken, which gives rise to some of the ways in which the map can be unfair. This is due to the middle land having some kind of ability to swallow anything that should be placed at its centre, and force resources towards its edge (or the edge of the map, if there are back pockets behind bases, as in some 1v1 spawns of Arena), and commonly clump them in strangely artificial lines and grids.
Long essay version, with examples and discussion of what is wrong with the map
All of the main versions of Arena share their main code regarding how the map is put together with the standard Arena that comes with the game. So all versions share the same problem. The extra resources (gold and stone which do not 'belong' to players) are supposed to be placed a certain distance from all players' TCs, and randomly scattered across the entire area which is at least that distance.
You can see in the picture above that ALL of the extra resources have gone into that little pocket on the right. This is because the min_distance_to_players for the extra gold and stone is 40 (so 40 tiles from the TC). That means there's only the far right side and far left side of the Arena available to place these resources on, because those are the only areas that are at least 40 tiles from both TCs. Still, the game had the opportunity to put some of the resources on the right and some on the left, but it chose to put them all in a small clump on the right. Note that there would be a much larger area to put the resources in on the far left side, if the game wanted to, but it still clumped them all together. It won't ALWAYS do this, but Arena definitely exhibits a strong tendency to clump its resources together.
In 1v1, this usually isn't a problem when the resources spawn in the arena, because they'll usually be off to one side, in neutral ground. The requirement for the resources to be at least 40 tiles from both TCs means the resources generally can't appear directly between the bases. However, as we all know, sometimes in Arena you have an extensive back area, the far reaches of which can easily be 40+ tiles from the TCS, and quite often the extra resources get placed in these areas behind a player's base if it's possible. Again, the game will show a strong tendency to clump the extra resources together, so quite often multiple extra golds will appear in these pockets behind players' bases. Contrast this to a map like Arabia, where the extra golds and stones have the ability to scatter across the map properly (though sometimes they won't, but that's working as intended according to the parameters defined in the map script).
In TG, the problem emerges more clearly, first of all because in larger maps you don't get the same extensive pockets behind player bases so much, meaning that the extra golds have to go in the middle. The player bases are generally on all sides, and the map is large enough that the extra golds can be at least 40 tiles away from everyone's TC and still fairly close to the walls. In this situation, what you'll see is that the extra golds, if they appear at all (we'll come on to this in a moment) will never appear particularly far away from players' walls - certainly not towards the middle of the map - and they'll usually be clumped together in a line or grid formation that meshes with the player golds, with fairly regular spacing. Often, this places too many golds too close to some players (or one team). As AoC players, we're used to imbalanced maps and we deal with it. At least the golds are not behind the enemy base in TG. But the middle of the map will be curiously empty.
So, what causes this clumping of the extra resources and an emptying out of the middle on Arena? It's to do with the way the map is made. The base terrain on Arena, unlike most maps, is forest, and the player lands and middle area are cut out of the forest and grass terrain is laid down. On most other maps, it's the reverse, with the base terrain being grass and forest terrain replacing the grass. There are two commands for this. One land is created for each player by one command. The other command places a middle area of grass which must stay a distance of at least 18% of a map edge from each border. This land has a slightly different grass than the grass used for player lands, you may notice. It is this land which causes the problem with the extra resources. The game tries almost as hard as it can to force the extra golds and stones to appear as far away from the middle of this land that creates the central 'arena' on Arena. This has an effect on where the extra golds and stones can appear. Quite often, they won't appear at all, because the game has no option but to place them near the middle area, because there is no area on the map that is at least 40 tiles from all TCs and yet isn't in the middle of the map (more precisely, in the middle of the land that creates the middle area). Take this screenshot:
The bright red rectangle is the only area on this map where the extra golds could appear, because everywhere else on the map is at least within 40 tiles of a TC. You will notice that there are no extra golds and stones in this area, because it's close to the middle, and Arena tries very hard not to put any extra resources in the middle of that land. It also tries to put resources into fairly evenly-spaced clumps in a line or grid formation, as I mentioned before. Here is an example of this occurring on another TG map where the player bases leave more area at least 40 tiles away from TCs, so that these formations of extra golds, stones and forage bushes appear outside of bases. Due to the clumping, all the resources have appeared next to three players on one team, nicely placed in one area which is mainly outside the walls of the pocket player on one team. If you divide the map into two halves, one for each team, literally every single one of the extra resources has been placed in one half of the map, with such regular spacing as to look almost deliberate. There was plenty of space on this map, where the player bases are mostly very far towards the edges, for a random scattering of resources, which is what should happen, to take place, but instead the map looks very artificial.
Here is an example of where I have modified Arena slightly to illustrate the problem better. I've made the middle land into dirt terrain, asked the map to place a lot more extra resources, and reduced the minimum distance to players. The result is massive load of extra mines appearing outside player walls in the conspicuous grid formations, but once again avoiding the middle of the map.
Now for the somewhat more technical part: how can we fix this, and get a random, broadly fair distribution of resources in the middle of the map?
It will presumably involve remaking the map without that curious middle land that seemingly has a black hole at its centre, or else adding some kind of property to it (and/or the resources intended to be placed upon it) Unfortunately, I'm not sure how this would be done without either introducing another problem or making the map stray too far from being Arena.
You could do it if you only had the middle area comprised of player lands instead of having a separate middle land which serves the dual purpose of creating the middle arena and giving players access to the middle. Without the middle land to join together the player lands and a command to then connect all the lands, you can't use other_zone_avoidance_distance to force player lands apart, because then the map would turn into Michi, as the zone avoidance distance would be filled in by the base terrain, trees. Without other_zone_avoidance_distance, player walls, given that you want them to be a reasonable size, can spawn too close together far too frequently, and will not uncommonly even go so far as to intersect.
Scripter suggested adding another land to the middle with a different terrain and forcing the extra resources to spawn on that terrain. But as soon as you add another land on top of the player lands and the one middle land, bad things start happening with the trees:
It gets worse. You obviously wouldn't want any extra gold to only appear in the middle. This isn't Gold Rush, you want your extra golds to scatter into the empty space properly. So, you have to make that land with the dirt terrain cover more of the arena - close to, in fact the same amount of space the original middle land that created all the problems occupied. So, suddenly we are back to square one, only with more than a single offending middle land, you can get this kind of thing going on. By making the dirt area bigger and asking the extra golds to only place on dirt, with a minimum distance to players enforced, the old problem has reared its head and the grid formation of resources with regular spacing, all clumped in one place where possible and avoidng the middle, has returned:
So, how do you solve a problem like Arena, and make those extra resources properly spawn into the right areas and scatter properly? I haven't a clue. But this curious behaviour created by the middle land is what has given rise to Arena's famed unfairness with back pocket resources in 1v1, and extra resources either spawning unfairly or not spawning at all in TG (due to the lack of back pockets or empty side areas).
Even if you just place extra resources for each player but try to make them spawn a greater distance away, they still encounter the same problems as long as you have that middle land and you ask them to spawn further away than right outside the walls (as extra resources should). But that middle land, as far as I can see, is the indispensable key to making Arena work as intended.