Can't seem to get S rank except when chests are off cooldown

Recently I have noticed that my game rankings for a champion have been much lower than when the chests are off cooldown. Just an example but I recently played a game where MF went 14/8/18 and Voli went 18/10/10 and they both got S- rank, i on the other hand went 26/9/22 , got two unstoppables, dealt the most damage and soaked up the 3rd most damage as Fizz, yet I get A rank (not even A+, just A). Where as when chests are off cooldown I went 8/5/7 as Garen and get S-. Can you tell me why this is? Is it a fault or just a weird way of working out rankings?