We know most Q-Ups exclude three-class equipment as a matter of course, or parks you in Z to prevent upgrades and seat assignments into F.

Maybe your ORD-DEN flight number is not a continuation of an inbound international flight to ORD, so the system is not auto-excluding it, or because it doesn't continue, UA doesn't put Z inventory on that flight.

Or perhaps it is a three-class moving to two-class or two-class moving to three-class per a schedule change or a flight normally operated with two-class that happens to be subbed with a three-class for a few days or weeks since it's more convenient to have the three-class plane for positioning or meeting F/C revenue bookings on the inbound/outbound connecting international segment, either via a through flight number on that piece of equipment or to support connecting flights with three-class equipment that this flight is ferrying passengers for.