Of course, there is always a chance that you make a blunder or misclick or get disconnected, but they are accepting that you had a winning position.
I choose to resign almost immediately in a rapid game where I am down a minor piece or more (unless I have major compensation). In faster time controls, like bullet and blitz, I am not usually quick to resign. In bullet, I will keep playing even when down a Queen or more, because I believe that I have a reasonable chance to at least hold a draw. There are techniques to create a fortress like position and run down the opponents clock by playing moves that cause the opponent to think.
It's very common in my games, 1100-1300 range. The player is very aggressive from the start and pretty much every move I'm on the defensive and they are attacking. Then they misstep and now I am up a knight or bishop, or even a pawn and I'm going to be the one attacking and they instantly resign. Their position is far from lost and my rating thanks them because they'd win or draw a lot if they played it out. It's just really weird that they do that. My best guess is that it's their bruised ego.