Champion Mastery

So I was wondering what the criteria for grades at the end of the game are. I am pretty darn good with Ezreal and have had a number of games where I have had scores like 10/1/9, 8/1/7, 10/2/5 and the like (you can see them on op.gg) with plenty of CS, yet I end up with an A- or a B+. I'm not trying to complain really, but I would like to know what part(s) of my game is(are) falling behind and causing me to get such low grades when I am doing so well.