Lots going on there. I will spend some time looking at it.
I suspect you will find the rear most part of the roof is narrower than the section right behind the drivers door so the pie cut is to deal with the mismatch of the B pillar width to the D pillar area’s width.
Most cars taper to the rear.
Yes, indeed - there is a taper - the cuts in this pic are to accommodate the taper. The pie cuts in that pic are just in the discarded outer skin, that's what's confusing - unless that is to allow a cut in the "outer" framing that is not documented.