Post game grading doesn't make sense

I'm wondering what the logic is behind the grading system. It seems that grades are assigned at random. My past 2 Rengar games I've gone 7-0-7 and 14-6-5. Both games I have farmed very well (180 cs from the jungle @ 30-35 min), taken multiple turrets, secured multiple dragons and baron, and won both games. I was rewarded for my efforts with a grade of A.

I'd like to understand why I'm getting A's for these types of games. I get that there is a comparison between my performance and the top percentile of other rengar players. But I'm just amazed that 7-0-7, literally a perfect game resulting in a win, was not worthy of even an S-. Also, I know 6 deaths is kind of alot, but when its balanced by 14 kills, a ton of farm, and complete jungle control of both sides of the map, I just don't understand.

Enlighten me.