Refunded only half the points I lost against a cheater?

I played a game today and lost 9 points. About an hour later I received a message that I was refunded 5 points because I played against a cheater. I confirmed it was the game where I lost 9 points (my opponent's account is closed now). Why didn't I get the full 9 back?

Makes sense. I should have searched the q&a first. I just assumed there was a problem.