STL-DFW takes longer than DFW-STL because of the direction of the flight. Wind (usually blowing from west to east) is the reason.

Most STL-DFW flights are blocked at 1:35 to 1:45 while the opposite direction is blocked at 1:45 to 1:50 or so.

I doubt that any plane ever flown by AA could make either direction of a 550 mile flight from gate to gate in just an hour.