Appreciate some help with this situation:

I booked two tickets for non-elite UA flyers about a week ago for travel beginning 6/20/06.

Traveler A (had a previously canceled flight, so I called UA and used the itinerary as credit towards this flight):

Outbound: 6/20/06 Flight 924, LAX-IAD, Z Class.
Return: 6/21/06 Flight 947 IAD-LAX, Z Class.
Total charge $928.60

Fare Details: CONXC 23APR06 FC LAX UA WAS 240.93V31NVDX UA LAX 603.72EBIZX USD844.65END UA ZPLAXIAD XT5.00AY9.00XF LAX4.5IAD4.5 SVC100.00USD.

The previous fare used as credit was higher so we are getting a credit voucher.

Traveler B (booked online):
Outbound: 6/20/06 Flight 924, LAX-IAD, Coach
Return: 6/22/06 Flight 195, PHL-LAX, Coach
Total Charged $928.60

Fare Details: C14JUN06 LAX UA WAS 240.93V31NVDX/-PHL UA LAX 603.72EBIZX USD844.65END ZP LAXPHL FARE USD 844.65 TAX 5.00AY TAX 63.35US TAX 9.00XF TAX 6.60ZP TOT USD 928.60.

Early Monday morning, I had to change outbound flights for both travelers to Flight 324 departing at 6:00 am instead.

CSR charges me $800 for traveler A, the fare difference that includes the $100 change fee. For Traveler B, there was a $200 charge that includes the $100 change fee.

All fine except Traveler A goes to check in and finds out it is economy class not business/first. Call CSR, no first class available, fine. When I ask why there was such a difference in fares where Traveler A was charged $800 and Traveler B was charged $200 I got an answer where the Z fare was so discounted that the resulting change in flight Traveler A had to pay $800 more, yet Traveler B, on the same flight and already in economy only pays $200 more. Please help me understand what I’m missing here. Was the Z fare really that much of a discount from economy?