i guess another important variable is randomness in civs. For ex, i always start the day with random civ with random unselected. Which civ i get is ofc random but soon enough i will have randommed most strong civs and then i sometimes get a bunch of weaker civs on arabia. This fluctuates my elo a lot, especially if there's a lamer civ picker queuing at the same time, in which case i have my preferred 2-3 civs like Hindustanis/Mongols/Meso selected, which further reduces the chances of randomming those civs