[Gameplay] Grading system has inconsistencies

So I understand Riot has kept the core game metrics which they use to determine your in-game grade secret, but whatever means they are currently using are flawed.

Let's look at this game for example: http://matchhistory.na.leagueoflegends.com/en/#match-details/NA1/2185619832/37151081?tab=stats

Here I, as the Rumble player had the highest gold, damage, dealt to champions, best KDA, kill participation, and in general performed the best on my team and carried them to victory. I got an A+ rank for this game, which I was admittedly salty about because I've noticed that when playing Rumble in the jungle regardless of how well you perform you consistently get lower grades than you would on other champs. This maybe due to the fact that I'm playing him in his 'secondary' role rather than top lane but the Riot grading system criteria should in theory be able to take this into account.

Earlier this week I had this game: http://matchhistory.na.leagueoflegends.com/en/#match-details/NA1/2180508464/37151081?tab=overview

I played Galio mid and IMHO I underperformed. I had about the same damage dealt to champions as the rest of my team, my KDA wasn't the greatest, I placed less wards than the Rumble game, and played rather poorly in general. I got an S grade for this game.

Now I because I got a S rank for a game where I felt I did not deserve it and didn't get it in a game I did deserve it I felt they cancel each other out in some way. But that's not why I'm bringing this to everyone's attention. Riots grading system clearly has some inherent flaws that come to the forefront in some very specific situations it seems (in general I think it operates fine), and I just wanted to bring that to Riot and the general players attention.

Has anyone noticed this with other champs? Specifically playing a champ in a different role than they were intended? Does this happen with many other champs? I've noticed it's common with Rumble in the jungle and since I don't play Rumble top lane I don't know if it's the same case there. It might just be that there's something specifically wrong with grading Rumble but I doubt that.

I first made this post on reddit which is why It's written in the manner that it is.