Interesting, but as the saying goes, 'we don't know, what we don't know'
Accelerators may or may be part of it.
Along with agreements with exchanges, i.e.can exchange  "A" always send out at a low fee since pool 'B" will always put it in their next block since they are getting BTC on the back end.
Or, does pool "A" not want to deal with CPFP or other things for some odd reason so they ignore all those TX.
And so on.

But still a nice site you setup.

-Dave