When you get an S and someone who didn't do as well as you gets an S+

Assists count toward your score, so although she had one more death, she had three additionnal kill participation over you. In fact assists are just as ''heavy'' as kills, so it does not matter if someone is 20/0/0 or 0/0/20 for score purpose.

But she does have less minions than you........ AND way less gold so yeah her S+ is weird.

It might also have something to do with scaling shrug