Busted Team Grade System?

So lemme first say that perhaps I'm still 'ignorant' about what it takes to be an S-Rank in the game of league. BUT, this makes no sense when a person plays well enough to earn it, and cannot get it. After dealing with some of the worst nonsense, for a whole 40 minutes, to be undermined.

Like lemme give you a run down of what these two screenshots will show. I not only had the least deaths ( from what I'm told anything over two disqualifies an S+ regardless ) to which I didn't die the last 20 minutes of the game, I had the most kills, 3rd highest in assist, the most damage to enemies ( 2nd highest overall ) and tied with my team for second highest in turrets destroyed. I did have the most CS next to Jinx of the enemy team, yes, but I wouldn't think that would count against me.

Now, as I said, PERHAPS, I am still a bit ignorant here. But it's odd. I've had games where I go 22/4 and got the S. Perhaps it's something to do with the roles? Or perhaps character difficulty? Someone has gotta help me out here. This is the 5th time, I've been undermined an S I worked hard for. Maybe the people over there at Rito can shed some light on this for the noob-kid. But I'm starting to feel the system is a little faulty.

Here are the screencaps to show everything I've explained for those who find this TL;DR.

http://i.imgur.com/cMdgw1u.png http://i.imgur.com/B8MfqsW.png