S ratings on Bot ADC Twitches

Asking anyone who has gotten (or continually gets) S ratings on Twitch as the bottom lane ADC. I've struggled with it, usually only getting as high as A...mostly I figured it came down to deaths since Twitch is squishy as hell. But I just played a game and was convinced I was going to end up with an S rating (yeah, I know. Shouldn't have jinx'd it) and ended up getting only an A- I mean..when I first saw I didn't get an S, i figured hey maybe A+...but A-?! I went 13 kills, 4 Deaths, 13 Assists. And got a total of 9 buildings destroyed. Granted, my CS was low, so I am figuring that is where I lost out. I only had 141 CS...but it was still the highest on my team. In fact it was the highest in the whole game! Also giving me the highest gold collected.

I am at a complete loss for how I somehow managed to not get an S in this game. It makes me wonder if Riot is fixing it so you don't get free chests, and have to buy them instead.

So yeah...I am looking for people who play Twitch Bottom ADC to give me some advice and let me know if I'm getting close or not. Thanks.

P.S. Here is a link to the match http://matchhistory.na.leagueoflegends.com/en/#match-details/NA1/2420166769/32626885