Grade System needs Tuning

I was Ryze 5 / 5 / 3 and i scored S Rank

Long ago I was Yi 12 / 0 / X and i Scored S or S-

in Both cases i won the games

So how is it possible for me to feed so much on Ryze and Score S Grade but when i perform Flawless on Yi to score something lower than S+?

How is that system working?