Champion Mastery Broken for Supports?

So I was playing around with my friends, and I did two games, one amumu support and one kalista support(I know it's bad, I just wanted to try it for fun). So for my game with amumu support, I did really well with my adc, winning lane hard early game, but our team lost all other lanes and we couldn't come back from it. I was, however, happy with the way I played, setting up many plays, defending turrets, and saving people. Then for the game with kalista support, I did absolutely terrible(As should be expected), whenever I roamed my adc died, could not keep anyone alive, and died alot myself; so we lost. However, for the amumu support game I got a C and for the Kalista support game I got a B. Maybe they need to change the formula or something? I have absolutely no idea.