This Makes No Sense

So I Played a Match in Hexakill and I did really well and somehow got an A+ and our Caitlyn got and S- Here's our scores: Me: 13/0/18 Cait: 20/8/14

Like, really? She only had 7 more kills I had 8 less deaths I also had 4 more assists

I'm just confused on why we didn't both get an S- because it is possible to have more than one S in a match. This just really confuses me, and I just want an explanation on how this is calculated.