Champion Grade seems arbitrary

So I just played a game. decided to play , cause why not? I am not a good player. I do my best and try to improve, and the grade system sorta helps with that.

By some miracle, I go 23/8/15. I get A+. gets S at 7/1/12 gets S at 11/9/26

Basically I participated in 38 kills and died 8 time. Karma participated in 37 and died 9 times. I understand that different champions have different play-styles, and so getting high stats on an easy champ merits a lower grade than high stats on a hard champ. But it seems arbitrary as to what determines that grade. And frankly it's frustrating.

Did I not cs enough? did I forget to buy a ward? I don't know and, as a player trying to improve, its infuriating to not be able to tell what I did wrong when I out-performed my team mates (not to speak poorly of them, they were awesome) yet got a lower grade.

I really wish there was a way to see what the parameters for grading is for each champ. It would help me, as well as other players, to better learn the roles of certain champs, track progress on our performance, and improve areas we might might fall short in.

Thats my rant. Tl:dr; can we improve the grading system?