How is the loser dumping out of game and giving me a defeat?

this guy “Grimstone” as I’m deleting him in late game says “I’m bored, GG though” and drops the game and system shows me as defeated. what BS. How is this happening?

@IaMaBooZeR apologies for this player. The best thing to do is report them in-game. You can also do it here by contacting support.