REDO THE MASTERY ON CHAMPS

Okay so a lot of games I play I sometimes end up doing legit the best, but no i get an s-...... thats not the only thing when im doing that good my supp is 8/6 s+ mid laner 9/5 S top laner 14/2 S, now for me 16/4 and the highest farm, most gold, and the highest damage and most towers down. and oh yeah i got the only dragon.so you would think i would get s or maybe even s+ but NO I GET A S-............... WHYYY WHEN I DO THE BEST. REDO THE SYSTEM PLEASE. so stupid.