They are PERFECTLY ACCURATE for what they represent. If the EPA allowed them to advertise their own mileage estimates then they would all design their own tests that give them better results and all the tests would be different and there would be no way to compare them. The EPA dictates the tests and therefore the test results and it's the same for all manufacturers. The fact that the Prius is more optimistic than others is based on the way the hybrid system works.
Perhaps you should actually read what the sticker says.
Here is the exact wording from a Fusion V6 sticker:
CITY MPG
20
HIGHWAY MPG
28
Vehicle Engine Information
2007 FUSION 3.0L DOHC V6, FUEL INJECTION, CATALYST, 6-SPEED AUTOMATIC TRANSMISSION
Actual mileage will vary with options, driving conditions, driving habits and vehicle's condition. Results reported to EPA indicate that the majority of vehicles with these estimates will achieve between 17 and 23 mpg in the city and between 23 and 33 mpg on the highway.
For Comparison Shopping all vehicles classified as MIDSIZE have been issued mileage ratings from 10 to 60 mpg city and 16 to 51 mpg highway.