One of the problems with the ranking system is that they do not actually take into account the specific requirements of each class to succeed and lump them all together.
Assassins are not there to farm and get assists all game. They get in, they kill, they get out, and that's the extent of it. I've gone 20-1 or 25-0 with some assassins, but because of farm and assists being lower I didn't receive any S. Why? I'm doing my job exactly as it should be. My personal opinion is that people at Riot's balance team have no idea exactly what assassins do, but it isn't complicated: Kill people, and kill them when they respawn. Prioritize carries and high bounties. That's the end of it.
However yesterday I had the pleasure of going top with Miss Fortune and going 27-0, taking Elder, Baron, at least four turrets and three inhibitors, a penta, several doubles, and near 400 farm. I got an S- and not an S, and since she's level 6 it doesn't count towards level 7. I mean, I can always get more S's but the real question is why someone with those stats doesn't get a regular S.
Oh, and if you are wondering why MF top - My opponent was a melee and had no idea how to stop kiting. Just what I expected!