Match ratings seem to be unbalanced

I was playing really well this match (kind of bummed that I was not able to keep a perfect score) and when we won and got to the results I was surprised to see that two of my teammates got an S rank, but I did not. My rank for that match was an A- which makes no sense to me. I have gotten S ranks with scores with less assists and maybe a few more kills or deaths, but really?! An 6/2/21 is an A-??? I already have a chest and level 7 mastery with sejuani, but it is not a good feeling to do really well in a match only to get a low rating. A few of my kills were taken from me, but I was okay with letting my team get the kills, since I did not really need them as much. I am sure other people have had a similar case where something like this has happened to them. One of my matches in the past I queued up with a friend as a duo bot lane and we destroyed. I got an S rank while my friend did not and I even helped them get a quadra kill that match. http://matchhistory.na.leagueoflegends.com/en/#match-details/NA1/2355774266/231996695?tab=overview