That means, coincidentally/cause of how random matchmaking is and depends who is online at the time, you are beating players lower rating than you, and losing to players even lower rating to you.

So if you're 1k, and you beat an 800 player, you might get 15 points, if you lose to a 600 player you lose 40 points, if you beat a 1200 player you gain 40 points, if you lose to a 1200 player you lose 15 points.

Random numbers but you get the point