Not sure if this is what you are looking for and if it is still up to date, but Documentation & FAQ gives some info and links to more info about rank maths:
The Glicko rating system and Glicko-2 rating system are methods of assessing a player's strength in zero-sum two-player games. The Glicko rating system was invented by Mark Glickman in 1995 as an improvement on the Elo rating system, and initially intended for the primary use as a chess rating system. Glickman's principal contribution to measurement is "ratings reliability", called RD, for ratings deviation.
Mark Glickman created the Glicko rating system in 1995 as an improvement on the Elo rati...
http://www.glicko.net/glicko/glicko2.pdf
tl;dr We’re switching to a Glicko-2 rating system, which should be great for a lot of reasons. All of your past ranked games have been accounted for during this transition.
Hello OGS!
For the past several years we’ve been using a rating and ranking system based on Elo developed by the European Go Federation. It has served us reasonably well, but there have been some shortcomings when applying that system in an online setting. Most of the shortcomings can be traced back to the fact that the sy…
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