So here's a pretty logical conclusion I came to:
Getting an S-, S or S+ is far easier with high-risk high-reward champions. (Yi, Tryndamere, Akali, Shaco, Yasuo etc... and half the adc pool)
The reason for this is because the grade system compares your statistics (such as damage dealt, cs or kill participation) to the average of the role you've taken. So since these champions tend to usually be at either extreme side of the scale, in games they've performed well they'll have a much bigger chance of receiving an S.
This is probably also the reason support champions are suffering from this the most - their kit is mainly based on utility, meaning they're always a "safe pick" that doesn't deviate too much from average effectivity.
(I'm not sure if the grade system also compares you to the average of statistics of the same champion - but the result remains the same. The low-end of the scale (meaning, games these champions performed badly) offsets the other end)
Solution: Based on statistics, champions that have SMALLER differences between "good" and "bad" games should have a bigger impact for smaller deviation from the average statistics.