How is this NOT an S - at least?

OK, Riot, I'm serious on this one. I played a game recently, where I was ADC, as Jinx. The game lasted 30 minutes, I got 10 kills, 0 deaths, and 12 assists, and I ended the game with around 200 CS. I say this, because 3 other people in the same game got an S rank, even though I had a better KDA than them, and pretty much everything I did was better. My kill streak, my cs, my kill participation, and my damage way outclasses everyone. I'll let your system off the hook for my support getting an S, because their grading system is WAY different than an ADC's, fine. But the Jax on my team basically AFK farmed for about 15 minutes (it was justified because I was destroying my lane, and everyone else was doing pretty well) and managed to get executed by a camp, but he somehow still got an S. Sure, he ended the game at a higher level, and he made about 1.5k more gold. In a 30 minute game that's not a lot.

Either way... yeah, I am kind of angry, but I just want to know why Jax got an S, and I didn't. I'm trying to get my mastery with jinx up, and hopefully gain another chest with her soon, but this system feels like it's rigged when stuff like this happens. So... if anyone has any idea why I didn't get it this game, I'm all ears - I want that S rank, and I want to get it by being good at the game, and if someone can tell me what I'm doing wrong, I'm all for listening.

Here's a link to my op.gg; it'll be my most recent Jinx game with the 10/0/13 KDA:

https://na.op.gg/summoner/userName=deloricvi

Figured I'd save anyone the trouble, because if you're going to try to help, you're probably going to go to op.gg anyway.