A while ago I claimed victory on one of my games and since then my rating has dropped with 30. How can my rating go down when I win a game???

Your rating is still provisional.

For your first 20 games your rating will bounce about as the calculations try to stabilize on a reasonable rating as quickly as possible. It's entirely normal ... just keep playing and as soon as you've completed your 20th game it'll go to the expected "win => gain points" method.

Thanks!