Rating points don't always go up and down by a stable number of points.
One of the factors is the difference in ratings before the game.
Beating someone rated more than 100 points below you only earned you 6 points while you lost 15 points for losing to someone rated close to your rating.
Basically what the title says. When I reached 1501 there seemed to be a glitch/bug where displayed my score as 1495 (in the stats sections, but at first it said I was still 1501 in the play menu) but I thought this was just the site being slow or something like that. The next day I checked my score again, but it had remained 1495 (this time it displayed 1495 both in stats and play). As you can see in this picture, I should have only lost 9-10 points but instead I lost 15. Any help figuring out why this happened would be appreciated
