Ceteris paribus, does it not make sense that the airlines with the most reserve aircraft and least congested facilities would be recording the best on-time performance? Right now, carriers like UA and US have more flexibility when it comes to aircraft assignments, and therefore should have a higher controllable on-time departures rating. While some carriers year-over-year ASMs are down over 10%, Southwest and Alaska are both operating at full schedule, so they will have more mechanical/crew scheduling delays. Quite simply, no airline is ever going to leave any more planes parked than it has to.
OK, so it's a bit of an over-simplification, but I don't think it's fair to say that UA and US are doing a better job serving customers by operating fewer flights. Besides, there are only six points separating the top and bottom airline here. Most likely, not even a frequent flyer would notice such a subtle difference. All airlines pretty much know what to do to minimize delays.