Getting an S

How in the world do you get an S? I've had games where i had half my team's kills, carried the entire game, or maybe won the game with a solo quadra kill. I've had games where i won lane, my team did well too, and we swept the floor with them enemy team. I've even had games where I didn't go so well early, but turned the game around in teamfights by blowing up the backline, finishing up the tanks with my team afterwards.

And none of these result in an S. I've heard that farm plays a big role in your score too, but this is, especially for an adc main like me, very reliant on the match up, and how good your support is, as in, if he walks up with you and pressures the lane. Also if the enemy team decides to group in your lane you are pretty much boned.

I understand that an S is supposed to be harder to get, but i think it's complete and utter bullshit that you can go something like 20/5/15 and not get an S just because you didn't quite get enough farm that particular game, or that you didn't go well in lane, but carried the mid to late game and ended up carrying your team, and not get an S because you didn't win lane. The ability to get back from being behind early, and winning the game, is just as 'skillful' as winning your lane, and snowballing the game with your lead, which is why i think the way grades are given out should really change, or at least should be explained better by riot, so you can actually perfect your game more, and not just pray to RNG that you get an S by any chance for ur 5th 20/5 game in a row.