Can someone explain this to me?

you downed 0 towers

The rank is tie to each champion, so 2 different champions performing exactly the same doesn't mean the same grading. Less people know how to play kennen so they end up doing worse than with MF

Not quite sure how different Aram is to Normals/Rank, but i would assume they are somewhat the same in terms of things like kda, cs, objectives, etc that are determined for the ranking. With that said to continue to what Rogue said. One problem i do see is that your CS does seem pretty bad especially for an ADC, granted again because it is ARAM the conditions could be different. But if that's the case, like what Rogue said less people play Kennen than MF.

I'm still looking for an explanation as to how a 5/12/8 Master Yi in ranked got an S-.

It's based on you region, what rank you are, what other people got, what lane your in, and what champion you are. So if everyone goes 27-0-15 on MF that might be like a B

Every champion has different metrics. To get an S you have to be in something like the top 2% of people playing that champion. What the top 2% is depends on the champion. All that says is there are more people popping off on Miss Fortune than there are on Kennen.