Hello, there is several threads about this topic already. here for example: Problems in rank?
Short answer (unless your case is somehow different) is:
- rating is recalculated after each game, but more than the last game (actually 15 games or one month worth of games if less) is taken into account
so most probably what happened is in some of those previous games you beat someone who then dropped severely in rank. In which case you still get points for the victory, but after next recalculation some of the previous points (not from the latest, but one of those previous games) are somewhat reduced, because the person you then played probably was not as strong as the system believed at that time (if that makes sense 
)