By observation, the rating system on this site is done by a very simple lookup table and not by the application of a statistical formula (although perhaps the lookup table had its origin in formulae). It seems to depend on two things: (a) your own BKR - not exactly, but instead where it fits into a few very broad bands, and (b) the difference between your BKR and your opponent's - again not exactly but in wide bands of 400 BKR or more. An inexact system like this will generate some odd effects but I think it's still fit for purpose.