Supports and the grading system

I'm a bit frustrated with the way grades are assigned. I just went 5/1/24 with Soraka, supporting our 20/0/6 Jhin. Considering how well I did in the match, I had expect an S- at the very least, but somehow only got an A, meanwhile Jhin was all the way up at an S+, 4 grades above mine (A+, S-, S, S+). Not a bad grade, but I anticipated a higher grade than that. All around I did very well, however I had very low cs. Being Soraka, this really isn't an issue, however my cs seems to be the only reason I didn't get a higher grade, maybe my kills as well. Perhaps I'm wrong, but if not, why the heck would a support like Soraka need to have higher cs or kills to get a good grade? This is a bit of a rant, but I honestly want to know how I go about getting an S on Soraka, do I really need to farm just to get a higher grade?