Crazy Grading System Sometimes D:

So I was playing Ezreal hoping to get my final Mastery 7 token, and I was positive that I had done just that with my stats, but instead I received an S- instead of a normal S or S+. It was a 37 minute game where I went 12/1/10 with 251 CS and 38 Vision Score along with only a bit more than 100 off from being the highest gold earned in the game, the highest was on my team. The previous S token I had gotten with Ezreal (A bit over a week ago) I was sure I had gotten at best an A+ with the stats that game but I received an S. Is their something legit wrong with the grading system, was it that I had too many assists or what. I'd like to hear opinions and see if anyone can explain why the grading system is so random sometimes. Just to throw in there, I once went 14/1/10 with Cho'gath top with about 170-180 CS in about 35 minute game and got an A+ so I kinda think too many assists might have something to do with it? I don't know why it would but idk at this point.