We play incredibly well and stomp the enemy team. I set up tons of kills, buy pinks whenever I can afford it, ward aggressively, even do golems when I get back earlier so my carry can last hit them on his way to lane.
Everyone else gets S.
I'm sure there's some logic to it. The problem is just this is what makes the grade system such a killjoy. You feel like you did excellent and then you enter the post-game lobby and everyone gets handed a big reward while you get... a B for playing support. That feeling isn't great, especially when you have no idea what else you were supposed to do to get a better grade. And maybe that thing just isn't supporting?