Mastery grades are too leinent

I feel like the only uncommon grade to get is S+ as I've only gotten that once in a game where I had a very good score, very high cs, and got a penta kill. I've gotten S grade from multiple games lost and have not even seen below a B. I have had teamates that some how got D's , honeslty thought it wasnt even possible.

Also the difference between a win and a lose in champion mastery level is far to big. I got two S raitings and a B but lost each of the games with the same champion. Then I got a single win with a B grade and had 3x as much exp to level than on the champion with better grades and more games.