Originally posted by MikeyObviously
Hi, I'm new to chess and the rating system.
I won a game as a 1265 against a 890, and my rating went down 20 points. Why is this?
Just to explain how, as opposed to just why, your rating for the first 20 games is an average of the games you play. When you beat a player, you get a score for that game of their rating plus 400. When you lose, their rating minus 400. The 400 becomes 200 for provisional (p-rated) players.
So your score for beating an 890 player was 1290. If you are saying you went down 20 points, my guess is that your opponent was a p890? This gives you a game score of 1090 for beating them. As this is lower than your rating, it will cause your average rating to drop.
By the same inconsistency, if you played me (rating 1856), your rating would go up even if you lost!! (your game score for losing to me would be 1456!). This is just an imperfection of the provisional rating formula. It sorts itself out when you get a 'proper' rating after 20 games, when this can never happen.
Hope this gives a bit more detail. As drew points out, the reason for this method is that the site doesn't know your true rating, and has to find the best system possible for estimating it, because the full rating system doesn't work on an unknown playing strength.