About the rating system

So I just finished a fun match with Swain, I did quite well as you can see, but I got an b+ for basically winning the game with Trynamere. He got an S+ and Yasuo had an A or A+ he said (I can't remember which). I think this is because I have little farm but Yasuo and Tryndamere took it all that is out of my own control. I could go for farm hardcore but if I do that I am not as useful to the team. My point is that CS counts way to much for your grade. I don't really care that much about the grading system but this is one of the weirdest grades I have ever received. Also what board should I place this in? Gameplay i guess?
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