S Rating System

So I am level 5 with ashe and have 1, S and need one more to be level 6. I have been trying for well over a month now. I have games that I do very well in and have no reason on why I do not get an S. I went 19/0/9, 185 farm, all turrets, and led every team fight and got an A-. How in the world is that an A- rating?!?! Another I went 7/0/11 in a 25 min game, with 166 farm and got a B+.... No way in the world that is a B+. Another I went 15/1/15 in a 25 minute game and got an A. How is that not an S? I don't understand why I am not getting an S when others can go 5/5/4 and somehow get an S!!! This whole S rating system is messed up!! I have picture proof of every single game and this is really making me mad.