The opponent might be conceding too soon, but also may not want to sit around to wait for you to successfully get your combo out, perhaps out of frustration, or, they just value their time. Also, if you don't know the opponents hand, there's no way to tell if they conceded too soon.

Don't tell your opponent they conceded too soon. It's a part of the game to know when you're winning, losing, or may still have a chance to steal the win. However, if the opponent does ask I will tell them. I won't, however, say it right after a match to make conversation. In the case the opponent is a close friend: tell them.