What's so confusing about the waitlist in the app?

It's pretty straightforward once you understand that it is showing F and PC together.

It is sorted by priority of who gets the F seat first. When there are no more F seats to allocate, it then allocates PC seats in the same exact order.

You may see people on the list who have a PC seat assigned; they are still eligible for F if it becomes available. You may see people with no PC seat assigned; they either have an exit row seat (which no longer gets auto-upgraded to PC) or have manually opted out of the PC upgrade. They too are still eligible for F if it becomes available.

And of course, as always, it only shows passengers who have checked in.