http://matchhistory.na.leagueoflegends.com/en/#match-details/NA1/2209750465/214906406
I got an A+ for this. My cs was terrible, but all of my other stats were good. I've had 40 minute games with basically the same stats (including cs) and have got s ranks, so I have no idea what I'm doing wrong.
There's so many hidden variables in this calculation.
To start with, it's looking at the performance of all champs in the same position, and the S grades only go to the top 4-5% or something. Maybe other Ahris are doing pretty well this week.
Besides KDA and cs, it also looks at wards placed, wards destroyed, camps taken from enemy jungles, kill participation %, how fair the match was MMR-wise, how much of a stomp it was, and then probably more stuff we haven't figured out yet. 160 cs at 42 minutes is fairly weak, and may have just gated you out of the S-tier by itself. It downplays kills on opponents of significantly lower level (the same way xp gains do), so if a bunch of kills were from you roaming bot & picking off an easy underleveled Ali, that probably has something to do with it too.
Mostly though, just try not to think of not getting an S as 'doing something wrong' but as just doing awesome when you do get it. At least that's what I tell myself after a dozen As in a row...
