If you forfeit, you don't get any XP for completing the round. If you pass and your opponent passes, then the round will be considered complete and you get your XP. So passing the round is saying "I surrender, but I'd like to get my XP please". Continuing to play in those situations is usually interpreted by the passing player as poor sportsmanship: "But wait, I really want to show off how badly I was going to destroy you before I'll let you get your 20 XP".
Think of it this way... When is the last time you saw a tennis match where one player injures themselves and walks up to the net to concede but their opponent refuses to shake their hand and spends the rest of the match firing unanswered serves across the net?
EDIT - I'm going to make one caveat to when I think it's not only acceptable but advisable to play a card (or two) after your opponent passes. That's if they're playing ST, have an un-flipped trap on the board, and the game is close enough that a Mahakam Horn playing for 8 points if you pass could decide the game. But that's not actually a situation when your opponent is conceding, that's them gambling on you being too sportsmanly to realize what they're doing.