performance is not just measured in kills. There are a number of factors.
In season 6 I was jungle main and got S about 50% of games.
Know your champ. - Farm, ward, hit turrets, - and yes, get kills.
As a reksai if you got 9 kills and only 5 assists, you probably were intentionally just taking kills when other people did a lion's share of the damage
(AKA stealing kills)
Sometimes the kills just fall that way. it's random who is the last person to hit a kill.
But who hit the kill last is also basically irrelevant as to who did the work. The killing blow was only ONE HIT.
I'd be surprised if the grading algorythm didn't take this into account.
Again, there are a lot of factors in your grade.