I fairly certain this rating system is due to recent performance of that champion accross the server. I think it is similar to the mastery system in World of Tanks. To get "Ace" mastery, you need to earn more experience per battle than 99% of the player base server wide during the past 7 days.
If this is true for League, you may get S or S+ for games with worse performance (less KDA) in other weeks or months on the same champion. I'm not sure what the metrics are, but I would imagine kill participation, ward stats, structure damage, CS, and KDA all play a role in your rating.
Again, just conjecture, but logical imo.