I Really Just Get An A-??

So I'm trying to earn my last S- atleast for my Mastery Lvl 6 with Lucian. Playing the game and we are really winning the game. I end up going 9/0/4 with 169 CS at 28 mins.... prolly not the great Cs but im only bronze 4. i was a part of atleast half of the turrets being destroyed. and had 50% kill participation. at the end of the game im excited to see an S- abut i literally get an A-..... hows is this even graded in the first place??