How Do I Get An S in ARAM? @Riot ...Maybe?

Been playing ARAM a lot lately and I had a couple of really good games. One was a 24/7/14 game as (Yeah, I'm not sure how that worked out either. I think they ignored me and went for Lucian) and the other was a 10/2/14 game as . In both of these games, I got a B ranking. Is it because I had 28 and 33 CS, respectively, in those games? That seems incredibly difficult and really unfair, especially if you are playing champions who don't really have wave clear, if you have another champion who does have wave clear, or if you are being nice and trying to let Nasus get stacks. http://matchhistory.na.leagueoflegends.com/en/#match-details/NA1/2250067692/214990377 (Urgod) http://matchhistory.na.leagueoflegends.com/en/#match-details/NA1/2251257452/214990377 (Caitlyn)

EDIT: More evidence for the theory. Just had a 18/5/43 match in Poro King (which is close enough to ARAM, I think) as with 46 CS (which was tied for the highest amount of CS on my team)... got a score of A+. There may be a different standard in Poro King then say, ARAM, but I don't how Galio with a score that high doesn't get an S. He doesn't always do damage and his spells have a long cool down, leaving it likely that someone else will often get the kill. His numbers should be lower, which means the CS has to be the critical thing, in a map where CS can mean very little.