S ranks and how they are calculated

http://matchhistory.na.leagueoflegends.com/en/#match-details/NA1/2204374046/227663546?tab=overview This is the match history page of my most recent game. i played trundle support. i went 12/3/23. I did not receive an s or even an A+. the game before that i went darius top 10/8/6 and received an S. There is a problem here. A trundle support at any elo should not get this score. It is mindblowing to me that i could only have received an A for this game.