Why is it so impossibly hard to get an S in a ff20?

With S-ranks being so precious now with Hextech and level 6/7 Masteries, can we take a look at how ranks are calculated for short matches? Getting an S when the opponent forfeits early seems to simply never happen to me or anybody I play with, and given some ludicrous scores I've seen, I can't imagine I'm having an isolated experience. My jungler friend just now had a 10/0/5 game in 20 minutes, and I can hardly fathom getting a better score in so little time, and yet he only earned an A+. Similarly, I had a game as Support the other day where I went 6/0/9 before the forfeit, yet still only got the A+. Conversely, I'll randomly get handed an S- for a good-not-great score in a long game, like going 5/4/33 as Support or something.

Yes, I know that grade scores are derived from more than KDA, but I think it's safe to assume that a 10/0/5 jungler had comparably high CS, objective control, etc.

As I type this, it occurs to me that as this becomes more common knowledge, early forfeits may even be a form of griefing. Other team slaughtering you during the laning phase? Forfeit and deny them their S's! Seems unhealthy for the game. Hopefully we can get a fix somewhere down the line.