Your mastery rating system has something direly wrong with it
I used to get a lot of S ranks on my other account when I was honestly not as good at the game as I am now, but on this account I've yet to get one S rank on shaco for mastery 7 over 72 wins and 54 losses, and to be blunt, great performances.
But I'm just a random nobody so I didn't put up a fuss but the jhin from my last match just went 12-0-11, almost highest damage dealt to champs, almost 21k dmg to objectives, 2nd highest was 14k.
They also had good farm, cc, vision, 4 towers and 1 inhib.
They only got an A+....
An s+ performance not even getting s-, bullshit, fix plz.