FIx The Grade System Please

So I was in a match just a few minutes ago and I was playing Kha'Zix Jungle. I assisted in killing 6 towers and got 119 CS with 16k gold. My score at the end of the game was 18/3/12 and guess what I got as my grade? I GOT AN A+

A few weeks back I was playing Shyvana and I carried that game with a score of 33/6/12. Something like that. I got an A+ too.

I don't clearly remember the Shyvana game but with this Kha'Zix game I was so pissed. I didn't go bot lane much to gank since it is a duo lane, is that why I didn't get an S? I also got objectives (1 Cloud Dragon out of the 2 dragons we got).

Please tell me why I didn't receive an S-, S, or S+