Vayne game: C- = 1.5k Mastery Points?

So I could blame lag or computer problems, but honestly I just didn't play well. 9/15/9 end score. Other team was ahead but ended up with not 1 but 2 AFK's, so my team ended up with the win.

I love the idea of champion mastery and the idea of accomplishment of getting to level 4 or level 5 (it gives a nice purpose to practicing a champion), and from my experience the letter grades dished out by the system are usually quite on point. This means champion mastery actually has some sense of each player's contribution to the game. So why on earth is the point system so blatantly focused on game outcome instead of the player's performance.

I get an A, lose, and get 100-200 points. I get a C-, win, and get 1500 points.

Those grades are accurate, so why can't the points be?

RITO PLZ