Simply put, the higher level player you beat, the more rating you get from the game. The lower the rating of the player you lose to, the more rating you'll lose. You may have lost to a couple of very bad opponents, or maybe some of your wins were against people with low ratings.
For more detail read:http://wiki.warlight.net/index.php/Ladders
Edited 2/1/2014 08:28:39