Originally Posted by HPN-HRL

I think Mike & Beckles have it essentially correct.

Having said that, here is another theory that may complement the above. Those of us who had fun playing "Are You Smarter than a Southwest Schedule Planner?" had to work with a handwritten "game board" for scheduling the planes. I wonder if the original scheduling department had to work with similar sheets to do the actual scheduling; if so, the lack of precision on the sheets would tend to lead toward choosing to schedule in the smallest increment they could see on the sheet (namely, 5 minute intervals).

Anyway, that's my "creative" theory...