Honestly.... it stems from nothing more than the existence of the XB1X.
Microsoft and leaks have confirmed that MS is working on multiple skus, but they haven't come out and said we will have the most powerful console on the market come launch. Everyone is just concluding that MS will d that because f what MS did with the XB1X.
I tend to disagree though. Mostly because I believe that MS and sony are going to get a very very similar APU. Pretty much identical with the option to have a slightly cut down version. Its not like MS is going to wait fr sony to release the PS4 then g t AMD and say ok.... they have 72CU in their GPU, we want one with 100. The differences in power will come from clock speeds.
What I really don't get is why people think MS will even dare come in at a price or option higher than the PS5.
Because they will have a cheap entry point system to complement it.