Originally Posted by
dkc192
It's been discussed elsewhere, but the prevailing theory is that this behavior allows UA to confirm PZ for those whom it deems valuable enough (high status, high fare class) while not opening it up for any Joe Schmoe with a handful of miles to spare.
I'm also fairly confident this happens, although I am unsure of the exact parameters. For example, I have seen multiple times GS/1K booked on OAR fares clear without even having P open.