Are grades decided by early game?

^title

Iv'e noticed on games that i snowball really early and get first blood at level 2 or 3 i will get an S grade regardless of how the rest of the match goes, if it goes average i'll still get an S as long as i go positive.

However on games that i do average for the first half and then start making fantastic plays later i won't receive an S grade no matter how much i do because of the first half, triple kills, quads, objective+kills, it just doesn't matter man.

For example this match i had i went 19/8/13 got 22k gold, had 291 cs and got an A+, i did meh at the beginning since i was against Kata and her jungle was very veeeeery close all match, so i played passively till i got a Maw to counter Kata's initial poop on you combo. From there on i was able to snowball, get fed, hit level 18 before anyone else and 1vs3 people. But no S grade v.v

What gives man how does this crap work now a days? halp Rito v.v