How grades are calculated

Question here, how com information from the graphs feel like they have nothing to do with what grade you end up with, for example, I played a sion game recently where I went 5/7/14, had the highest damage on my team except the assassin, had the second largest cc score, the enemy support having more, Took the most damage, beside the ekko who went 7/11, although my turret damage scores are low, I got a C+, how does that make sense?? Of course there are other instances where such things happen, but I don't want this post being too long.

Note that this is a genuine question of whether stats have anything to do with grades. And if it doesn't, i think it should, and not be centered around kda.