For example the Prius drops from 60/51/55mpg city/hwy/combined down to 48/45/46mpg. Similarly the Honda Civic hybrid goes from 49/51/50 mpg to 40/45/42 mpg. Not all hybrids dropped as severely with the 2007 Ford Escape Hybrid went from 36/31/34 to 31/29/30 mpg. Since all hybrids are highly dependent on control strategies that are defined by software, they can be tuned to perform their best on specific circumstances.
What this seems to indicate is that Toyota and Honda tuned their strategies to work best on the EPA test cycles to the detriment of real world performance. On the other hand, Ford seems to have tuned their system to provide the best efficiency to the driver. That's not to say that the Toyota and Honda don't still provide outstanding efficiency, because they do. It's just that the previously published numbers are artificially inflated, perhaps in an attempt to get bragging rights. I'll have more to say on this subject in a piece following my upcoming review of the 2008 Escape Hybrid in a few days.
[Source: Environmental Protection Agency]