Each airline refuel their planes differently.
Airlines try to fuel the aircraft for what's needed for the flight plus a little extra for contingency. Flying with a full tank on a short flight will reduce the fuel economy and at times, make a plane too heavy to use certain runways.
It's a delicate balance. Sometimes they get it wrong.
Also, each airline pays a different rate for fuel so united may have had a higher rate than southwest and opted to "roll the dice" They lost.
Also, just because 3 planes fly the same route doesn't mean they fly THE SAME route. Flying 1000 or 2000 feet above or below another flight could affect the winds. A flight corridor can be as much as 10 miles wide so again, varied weather patterns. Also, the passenger and cargo load was probably different. And lastly, not all 737-800 have the winglets which help inprove range.
I stayed at a Holiday Inn Express last night