Rating Question

This rating system seems strange, but I'm sure there is some rational mathematical explination.  In my last game White was rated 1311 and I was rated 1310.  When I won, I gained 11 points and they lost 8.  It just seems strange how my rating was 1 less at the start of the game and then 18 higher at the end.  Does anyone know how this works on chess.com?  I heard it's different than FIDE.

The rating variance isn't only to do with the difference in rating, but also with the 'certainty' of that rating (which is measured by considering how many games you've played recently).  Your opponent has likely played more chess than you of late, so the game had less effect on their rating when compared to their larger recent body of games.