Is there a higher % chance to get S+ for a sub 20 min game?

because.. we won in 19:18.. and FOUR of us got an S+

i do know the other 4 players were a 4 man premade but still

http://i.imgur.com/mautyMX.png

and i don't think ANY of us DESERVED an S+...

this is a link to the game http://matchhistory.na.leagueoflegends.com/en/#match-details/NA1/2268614697/207008136?tab=overview

idk if i should post this in the bug report section.. or not.