That's horseshit. I have games where I've landed >90% of all my hooks on Blitzcrank, go 8/0/18 as support in a 30 minute game, deal more damage than my jungle and top and comparable damage to my mid, and it still only gives me an A+. Their metrics are shit. I find it hard to believe that there is some large enough percentage of support Blitzcranks out there that are carrying harder than me at that point.
This Karthus got a penta, almost 2
playing 3v5 is fun
Pretty sure the ranking system has a lot more depth then people give it credit for. IT could be based on role, champion, damage dealt, damage taken, healing, ect. The grading system might even shift based on average champion results.
All in all, its probably very detailed.
A detailed system doesn't mean it's an effective, or even an accurate system. For all you know they could just be using a weighted RNG system seeded by the KDA ratio of a Bronze player in Korea divided by the number of Ardent Censors purchased in the last hour on the Oceania servers, and you would be none the wiser. Because it's all hidden behind a curtain, and the results are anything but predictable, let alone logical at times, it breeds animosity towards the system.