short trips kill fuel economy in any car. short trips is not the same as the EPA "city" cycle.
(BTW: for about the first 5min (or longer) of any trip from a cold start, the Prius will run its gasoline engine to warm up emissions componets and such to proper operating temperatures, so if you have a short trip where you don't get very far out of the warmup, you'll have poor fuel economy.)
I'm not sure if the EPA city cycle is the Urban Dynamometer Driving Schedule (UDDS) aka LA4 (city test cycle):
http://www.epa.gov/otaq/emisslab/methods/uddsdds.gif
or if it's the Federal Test Procedure (FTP):
http://www.epa.gov/otaq/emisslab/methods/ftpdds.gif
(Highway is: Highway Fuel Economy Driving Schedule (HWFET):
http://www.epa.gov/otaq/emisslab/methods/hwfetdds.gif )
As taken from:
http://www.fueleconomy.gov/feg/info.shtml#estimates
<blockquote>
How are fuel economy estimates obtained?
The fuel economy estimates are based on results of tests required by the U.S. Environmental Protection Agency (EPA). These tests are used to certify that vehicles meet the Federal emissions and fuel economy standards. Manufacturers test pre-production prototypes of the new vehicle models and submit the test results to EPA. EPA re-tests about 10% of vehicle models to confirm manufacturer's results. The vehicles are driven by a professional driver under controlled laboratory conditions, on an instrument similar to a treadmill. These procedures ensure that each vehicle is tested under identical conditions; therefore, the results can be compared with confidence.
There are two different fuel economy estimates for each vehicle in the Fuel Economy Guide, one for city driving and one for highway driving. To generate these two estimates, separate tests are used to represent typical everyday driving in a city and in a rural setting. Two kinds of engine starts are used: the cold start, which is similar to starting a car in the morning after it has been parked all night; and the hot start, similar to restarting a vehicle after it has been warmed up, driven, and stopped for a short time.
The test used to determine the city fuel economy estimate simulates an 11-mile, stop-and-go trip with an average speed of 20 miles per hour (mph). The trip takes 31 minutes and has 23 stops. About 18 percent of the time is spent idling, as in waiting at traffic lights or in rush hour traffic. The maximum speed is 56 mph. The engine is initially started after being parked overnight. Vehicles are tested at 68 F to 86 F ambient temperature.
The test to determine the highway fuel economy estimate represents a mixture of "non-city" driving. Segments corresponding to different kinds of rural roads and interstate highways are included. The test simulates a 10-mile trip and averages 48 mph. The maximum speed is 60 mph. The test is run with the engine warmed up and has little idling time and no stops (except at the end of the test).
</blockquote>
Chances are, you don't drive like the tests...