What the heck, Riot!!!!

So I did a Pvp match summoners rift, it was a really epic game and I did really well with Talon. (35/4/8) But my real problem is I didn't get an S of any sort, I got an A, not even an A+. While Brand got an s- with 17/10/6.... This makes absolutely no sense because I basically carried my team, and go literally nothing from it. I'm not sure what algorithms are, but it doesn't seem fair...