When flights are under airport control things often change wildly. Over the past few years when in J on a number of 737 flights sowing half full (and with sales availability still showing, so one presumes not just unassigned seats) close in to departure I've seen a seat block next to me (yes, in J) only to, 9/10 times, have all J seats taken by the time the door is closed. Often with staff standbys/buddy passes or last minute upgrades or changes from other flights - specially on your trunk/triangle routes (less so on more leisure routes like CNS-BNE or OOL-MEL) - all of which makes perfect sense to me (though yeah a few times I've sat there thinking you ripper when I'm sitting there on board to see the seat map suddenly fill).
I do think though this is less about P1 status. I mean when agents are trying to get a flight out, get standby pax and or staff on and so on in a small amount of time they don't(I imagine) often have the time of luxury to even bother seeing who has high status and whatever and just plonk pax wherever - a seat is a seat.
The case of te dash8 flight where the pax had come from PER seems a bit odd though.
I guess once I check in online and if it matters to me I do look at the seat map from time to time, and have been known to proactively change to a "better" (seemingly) seat.. sometimes that works out.. sometimes it doesn't.
I do agree though if you're in row 7 and you see seats ahead free(that aren't just middles) you'd think well what's with that? and I have no real explanation.
Now who remembers the good old days when NOBODY had pre-assigned seats on domestic and the computer did it all the night before travel and you got what you got? 
