If you buy a car that can get up to 50 miles per gallon, do you think you will actually get 50 mpgs? Have you ever driven in Los Angeles rush hour traffic? No you haven't. No one drives in LA rush hour traffic. You just sit there on the highway idling and burning fuel. Would you expect to get 50 mpgs then?
In LA you can. An LA judge has awarded a woman $10,000 in her suit against Honda because her Civic Hybrid did not get 50 mpgs.
So why would Honda put such an outrageous claim on their car? Because the EPA told them to. According to the judge, that was an insufficient reason.