Southwest's airplanes aren't quite identical. They fly 737-300's and -700's. The 300's are a bit smaller, with a lower service ceiling and a shorter range. A

Southwest's airplanes aren't quite identical. They fly 737-300's and -700's. The 300's are a bit smaller, with a lower service ceiling and a shorter range. At present, the 300's also have older avionics. They are not completely interchangeable. Still, maintaining only two types of airplanes that are very close to the same size and specs is a lot easier than maintaining ten types of airplanes with very different sizes and specs. The extra expense of sending a 700 on a flight that's only full enough to require a 300 (because they need a 700 on the other end) is much smaller than, say, the expense of flying a 747 for a route that only needs a 737. The extra expense of bumping one or two people (and giving them free flight vouchers) to free up a slightly larger plane for a different route is also quite small. As you correctly note, there are a lot of efficiencies that come in to play from having a nearly-uniform fleet.