-1You are probably right; most people can get 1800 with good training.
But it doesn't mean all 1800 are equal. Some people get 1800 only when they are in good form, rest of the times they are lower, some other are always 1800 but never more, and for some, 1800 is just a transit to higher.
Progression isn't constant. Sometime you stay at the same level for a while because you are "stuck in your ****" and some day you star changing things (instead of trying to perfect wrong things) and progress again...
But rating means nothing. It depends on civs, maps, on if you are tired, rusty, oponent's style.... and luck.
Just play this game for what it is and not for the ratings!