They are basically treating original fare as a sunk cost with the FCM upgrades and assume people only look at upgrade cost when deciding whether or not to purchase upgrade (and don't really factor in what they originally paid for fare). With someone flying for business where employer pays for original fare, this is literally true. With a leisure flyer who paid themselves, the original money they put in still has value but DL is likely assuming most people don't really look at that and fare differentials. And if DL can get them to pay more for FCM upgrade purchase than paying fare difference price, it's a win for DL. Convenience is likely a factor here since you can just select a seat with the FCM upgrades while paying fare differential requires you going back through change flights, and reselecting flight which is not so convenient/obvious to people.