How does the system of Mastery works? Because I think is broken
So, I just finished a game where I'm sure as hell that I did almost perfectly and deserved an S at least but no, to my surprise is an A+ so how the hell does someone who is far ahead of the enemy team and their teammates with a 12/2/4 score and with 118 of farm doesn't get at least an S, what do exactly have to do, farm more? The heimerdinger didn't got one even if he was the best with farm, Jhin neither but the one who got an S was the Sona so I'm not particularly mad, just really confused because it seems that you only gain some because of the rng more than based in what do you do in the game.
Please can some kind heart can guide through this confusion [zombie-nunu-bummed]