OK what the hell is your scoring system Riot?!

OK, I just finished a match, started Malzahar mid, counter picked katarina, the whole shebang. It is going amazing I am winning lane the jg has failed 2 or so ganks on me and I am fed as I start roaming around and getting kills, helping the team with turrets, yada yada. I end the game with this score. 16/1/4, 172 CS, and 13,611 gold earned. My end score is A+. I also did a little over 30,000 damage to champions, the next best was a jinx at a little over 15,000. So what I wanna know is why the hell I only got an A+, I have been told multiple times that the requirements for a mid lane S, grudgingly, is that if you die more than two times you lose the S, and this has remained to be true until the last patch, I have gotten better scores with multiple different mid laners, from Ahri Talon and even Taliyah etc. And this keeps happening even if I don't die at all and it is starting to get maddening, the one thing I wanna know is if this is a bug, or legitimate requirements to get an S, or numbers we should aim around something so that we aren't confused or angry or anything like that. It is one thing that I don't like about playing this game is that everything outside the actual game is so inconsistent from the looting system to your scoring system, your client even breaks down sometimes! But the game is great and it is the only reason I really stay here, I just want some more concrete stuff so other players and myself can enjoy it to its fullest extent.

Thank you for your time, A fellow gamer.