My rank decreased after a win / My rank is behaving weird
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Long story short - the rank calculation is a complicated process our developers spent a long time on. It may be doing something strange at first sight, but we are pretty confident it is working more or less correctly.
The main source of confusion is that the ranking system is constantly recalculating your rank based on how the rank of your past opponents changed (to the extend of last 15 games). This is a good thing! But it may in certain circumstances even lower your rank after you have won another game (you always get the points for the win, but some of the points you have gotten previously can be decreased).
Say you have played a 5 stones stronger player in the recent past and beat him. You get a lot of points for that, it is after all a big acomplishment. But later this player’s rank fell a lot. That probably means the player was not on the level the system thought and the boost to your own rating might have been too much. Therefore some of the points given to you for that victory can be decreased retroactively. It may feel disappointing at first, but it helps a lot to get the most precise estimate of your actual rank.