I recently booked TATL travel that showed no PZ inventory, and when I applied PlusPoints also indicated this would be a waitlisted segment, however the upgrades were confirmed within about 5 seconds of request. Is there some sort of secret upgrade inventory? This makes route selection more difficult (which may be the point).

My only other clue is that the upgraded segments all had substantial PN inventory (which I take as a proxy for not having GS ahead of me on the waitlist). Perhaps there is something that automatically converts PZ to PN inventory under low-load circumstances?