A FlyerTalk Posting Legend
Join Date: Sep 2009
Location: Minneapolis: DL DM charter 2.3MM
Programs: A3*Gold, SPG Plat, HyattDiamond, MarriottPP, LHW exAccess, ICI, Raffles Amb, NW PE MM, TWA Gold MM
Posts: 100,417
Start by looking carefully at the fare code and fare rules. My guess is that the $3900 is a Z fare and $10,300 probably C (maybe D, but probably too cheap to be FULL J). Also, are you searching for *two* people in both cases, as there could be one ticket remaining at $3900 with the second passenger on the same PNR pushing the price up to $10,300 for each person.
Are you calling AmEx or using their website? I would assume/hope that they use real time inventory for DL, but that could also be an explanation. Maybe the cheaper fare won't actually ticket.
IME IAP offers discounts on fares that are normally available from DL (same fare code, just a designator), but I've seen Z fares that AmEx doesn't see and can't ticket under the IAP.