S and S+ ranks are not possible for me.

So, I play League every day. Anytime I play, it is with my friend Hayeh. While I acknowledge that he is better than me, there are of course times where I have a better game than him. When this happens, however, he still gets a better rank. He constantly gets S ranks while I get A's and B's. Even when I have a better game, he will receive a rank or two higher than mine. Take this game, for example. http://matchhistory.na.leagueoflegends.com/en/#match-details/NA1/2095364007/219915143?tab=overview
I was Yasuo , while my friend Hayeh was Gragas . I have more kills, more CS, 1 less death, more gold, I solo'd the baron and helped with every dragon and Teamfight if possible. I also had the most damage dealt in the game, to both players and minions. Looks like a great game, right? Both me and my friend carried our random teammates pretty hard. Well, at the end of the game, we both look at our ranks. Gragas (Hayeh) got an S+ rank. This excites me. "Well if he got an S+, surely I did as well" I thought to myself. Wrong. I got an S- How? I do not understand why I did not get the same rank. Maybe you guys have a reason and could explain to me? I'm completely out of ideas. I'm not the type of guy that hunts for S ranks, but sometimes I feel cheated. If anyone has any idea as to why he got a better rank, please let me know. Thanks, From A Confused Summoner. [slayer-jinx-unamused]