Champion Masteries Rating?

So I am a jungler main who recently started to actually pay attention to the match grade at the end, but no matter how well I do, I find that the highest I can get is S-. Is there an open formula for this grade, or is there something rito cares more about?

Take this game of Shyvana for example: I was playing norms with a friend and we basically carry the whole game. I had low assists not because I ksed but because the rest of our team just didn't do damage XD. And yes, this is low elo that's why we have a supp who really wanted to play garen and a eve jg who forgot to take smite.

Oops @.@ forgot to mouse over the grading but it says S-.