The ride home was definitely better. I opted to loop around the south side of the airport and add a couple of bonus miles for the return leg.
Garmin offered me an update to firmware v3.2 when I plugged the gadget in to GarminConnect web site. The release notes say it improves the accuracy of the Ascent/Descent reporting. Let's hope so. Mine always seems to read higher than other riders on the same route when I have someone to compare to.