Booking a flight in Y for DEN-Belfast for the spring. Did an advanced search, clicked the box for using a OnePass upgrade both ways.

The fare for the 10 or so options (which range from DEN-EWR-BFS to DEN-ORD-EWR-BSF) is exactly the same. But in some itineraries, the upgrade is available EWR-BFS, and in others the upgrade is stated not to be available. There is only on flight EWR-BFS (CO0094), so the only variable in each itinerary is the inbound flight(s). It seems like changing the inbound changes the upgrade availability on the TATL leg.

How can this be?