Questions about getting an S.

I'm pretty confused about how the system works. I currently need one more S to get level 7 on Rengar my main. One game I go 15/6/10 with 127 cs I get a regular S. I go 15/6/4 the next game or gamer after that. I only get an A+ which I understand because of the lack of CS. But then a game or two after I go 15/5/7 with like 200 or so CS, I receive and S-. That game was extremely discouraging and I'm so confused about how the rating system works. I understand some of the factors but I wish there was an official post about this (Not sure if there is or isn't but I searched a little). If anyone could clear up some info for me I'd really appreciate this. Thanks