I see no MIA-DFW on the schedule now. I've seen them on ORD-DFW in the past but not now, either. These domestic legs are simply positioning of equipment. We have a 777 between JFK and MIA starting up Dec. 18.
The ORD-DFW 777 was terminated because at the time they needed a standby plane at ORD in case of mechanical issue that would take a plane out of service.. So, the plane which had been based at DFW was moved to ORD. And, as you said, the domestic legs were just for repositioning.