Champion mastery grades are flawed?

I just played a game as support Leona and I went 0/1/11 with 30 creeps (team score was 21 to 4) and I got an A-. One of my friends in the same game was playing Teemo top and went 2/0/1 with 151 creeps and got an S-. My ADC (Jhin) went 8/1/5 with 114 creeps and did not get an S (probably got an A, but I don't actually know, he was a rando). This seems to me to be very inconsistent and flawed. I don't feel my friend deserved an S-, I feel like I deserve at least an A+, and I feel that Jhin deserved an S. I'm not sure where the consistency is with it. Does anyone else have problems with the grading system? (And yes, I'm a little salty cuz I'm trying to get rank 7 Leona)
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