Actually, I believe the short trip MPG has more to do with how long it takes to reach closed loop.

In a longer trip a greater percentage of the trip is done in closed loop, thus increasing your average MPG, since the computer can adjust the majority of the fuel used during the trip based on feedback from the o2 sensor.

In a shorter trip a greater percentage of the trip is done in open loop, thus decreasing your average MPG, since the computer is injecting the majority of the fuel used based on a preset map, without any feedback to whether or not the mixture is rich or lean.