I just played Tock, Table 60810391. I won, and got 10 ELO, rasing to 190. My opponent lost 21, dropping to 165.
So, before the game, we were basically on the same rank. That's one reason why our ranking should change by the same, medium value (around 15). The other reason is: that should be always be the case. When I gain X points in a 2 person game, the opponent should lose the same amount. And X is determined by our ELO difference before the game.
In Games with 3 or more players, X can vary, but the sum of all X should be 0 (with some room for rounding).
Or is there something about ELO that I didn't understand?