Do you have similar experiences with the Performance Ranking?

First off, I dont want to barg about things, I just want to know how it works! I know how the system should work in theory, but since the results dont match, I want to dig deeper and understand it. And this was made easier since we have more statistics now.

So can you guys tell me why I got an "A" in this game:

http://imgur.com/EIOqJ8u http://imgur.com/Slu3AM8 http://imgur.com/jlyzJFm

I'm not expecting to get an S or even an S+ every game, but this performance should definitely be better then just an "A". But I got an "A" and I want to know the reason behind this! Also remember that this was only a 17min game and Yasou and Xayah got both S's. So its definitely possible to get them. Yes, I know getting S's depends on the average champion performance, but lolskill says my performance was 93% better than the average Sion. Yasou and Xayah: 31% and 5% better than the average.

Let me know if this happens to you too, or if you have an idea why!