Increase in 737 bodies in DFW might support more 737 flying which would lower the junior lineholder from it's current 7500(?). Or more senior DFW 737 guys are switching to other seats. Back filling with junior guys from MIA would also lower the junior lineholder.

And yes, MIA 737 might back fill as guys bail to other bases.

That's why the junior bases typically stay junior, as soon as a guy gets some seniority they bail to a base closer to home.