Can Someone Explain The Current Rating System To Me?

I am very, very confused about certain facets of the current rating system.

Here are two games I played in recently. During one I laned well and won my laning phase decently, and tried hard to but couldn't get anywhere near fed enough to carry the rest of a team which lost heart and surrendered - that's the 2/3 game. During the other I ended up botlane during a set of blown lane calls, and utterly annihilated the lane with my laning partner, and ended up 8/0 en route to an eventual 13/2, with the two deaths coming because I couldn't ping others out in their greed and tried to save them - by far the game's hypercarry.

I get the exact same rating both times?

I know the rater actually tries to take situations into account, and grades you more highly if you do well in the face of a tough situation, and has much higher standards for a winning fed-botlane carry game than a toplane game where your team throws and you try hard anyway. And I'm flattered it gave me an A on the bad game. But I'd much rather have had a B+ (or worse) on the bad game and an S- on a 13/2 game where I was the undisputed hypercarry, particularly since my support was a main reason that I had those late-game deaths...and he got an S on the game. (Yes, he helped me get to 8/0, but he also refused to retreat no matter how low in health he was, and trying to be a nice guy and team player I went back to save him twice in situations I shouldn't have, which undoubtedly cost me the S.)

The only reason it matters at all to me is that you do have to get S games to get mastery fragments. 13/2 and a hypercarry seems like it ought to be enough for an S-. And there's no way I can get enough assists to please the matcher on a game like that - my teammates just didn't kill things often enough.

This has undoubtedly been discussed many times before, so if anyone has links to relevant threads, hit me.