It's very likely they were a new player with only a provisional rating. Normally the first 20 games are considered provisional, until a player has played 20 games of that particular type of variant. I believe Chess.com uses the Glicko system. So, in short, the early games for a new player swing very large to try and get them to their true level as quickly as possible. The more games someone plays the tighter the rating swing will be. Of course, other factors come into play such as their opponents rating, etc.
Hope that helps!
How is possible? My opponent gained 109 rating points after winning against me. He should have only gained 9 points. He is now 700 rated and I don’t understand why this happened does anyone know??