This is because your opponent must have a newer account, or haven't played that time control in a long time (90 days I think? Either that or 30 days; I forget which chess.com considers "active."
https://support.chess.com/en/articles/8566476-how-do-ratings-work-on-chess-com
I just played a game and lost. I was 938 and lost 7 pts. My opponent was 989 and gained 13 pts. WTF?