X2 what wing zealot and a couple of other folks have mentioned. Torque management is at work here. The first time I towed anything with my 2004 F-250, I noticed it first hand. I moved a friend's empty toy hauler from the rv shop to his house. the empty weight was very light, about 4,900 lbs, GVW was 7,500. I drove with the same light foot I use when my truck was empty on city streets, and my rate of acceleration was indistinguishable from when the truck was by itself. The software seems to sense that more weight is being moved, and cranks in more hp/torque to compensate. I also notice the same thing when towing an open car hauler with car, at a touch over 7,000 lbs.
