DFW is not the only market they serve. They have a large and diverse fleet that has to be scheduled to all points of the route-map and get maintenance at regular intervals in between. Like Greg said about the cargo, there are plenty of factors (hundreds actually) that fleet & route planners have to take into consideration to get the most appropriate usage out of their planes.
It could be as simple as "whatever bird is left after the more important routes get theirs" or as complex as a computer algorithm that schedules a whole month's fleet usage based on everything from catering availability to crew rest requirements to predicted winds aloft to advance F&J bookings and a thousand more things.
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