Originally Posted by
Kmxu
Bold face is mine. Evey response missed this point.
When one applies for PP upgrade, United indicates that upgrade is either available or waitlisted. There is no ambiguity in this.
I guess that OP received an email about the clearance of the upgrade almost right after he/she finished the request. Because of this fact, OP suggested whether or not there was a hidden PZ space somewhere. My answer is no. PZ space was released at the moment when OP made the PP upgrade request.
I'm not sure if United always displays PN space for one leg when not available across the full itinerary. I've seen it display PN4 for one flight and PN0 for another and other times I've seen PN0 for both legs even though one might display PN2 when searched along (the latter example I explicitly recall from booking international itineraries). So may just be an IT glitch that has progressively been fixed over time.
To your point though I wonder if it was just coincidental timing when they did a sweep/release or if there are certain triggers (based on low demand and time until departure) that cause PZ space to be created and released.