I think they understood that they were wrong after you have clarified the situation, but they're too stubborn to admit it, so they kept telling you that you're wrong.
In such cases, don't hesitate to quit from the argument and the game

Edit: I found something interesting on
wkb:
Quote from: TerminologyLeader - First, Top - The team that is at the top during the match, usually the one that is with more health (the "winning" one). However, if all teams have the same amount of health, the leader is still the one at the top.
But that doesn't make much sense, it's not even fair... Because the Red team is always the leader everytime a new game starts (only if team colors weren't changed)