Champion Mastery

I'm just curious how champion mastery works, I main jhin and a recent game i had with him i went 16-2-20 with 233 cs. We did win and even though my cs was lower than normal it was still a fantastic game. However, I was quite angry to find out post game that i only got an A- for a score. I've had many experiences with this game feeling very unrewarding and this just adds on to it. The fact that I'm slowly starting to fully comprehend is that league does not reward you for doing a good job. There is no reason that this level 6 mastery should require an S- and make it nearly impossible to get an S- when you do good.

PS. anyone who mentions i should try to do the champion in support to have an easier time getting an S. I have done this to no avail and that is not a successful system where in order to receive a good score you have to play a character a certain way different to how it is normally done.