Some of it is driving habits. Proper tire inflation is another. Load is still another. Weather conditions. Type of roadway driven. The cars are tested with a driver, new engine, perfectly inflated tires, on perfect roadway in perfect weather conditions. So yeah, mileage might be a lot lower for John Q. Public.
Can't speak for others' driving habits, but I drive briskly, and I believe in the "over 9, pay the fine rule" (that is, I drive about 8 mph over the speed limit in most places). Terrain around here is very hilly, borderline mountainous. It's part of my every-day driving.
The new engine used in the EPA test should get worse mileage than one that has a few thousand miles on it - that's been my experience anyway. As for tire inflation, that is a significant factor, but you'd have to be running at about 10 PSI to get the results that CR got. I'm not very trusting of the EPA in general, but I'm even less trusting of Consumer Reports. The friend of Nader is my natural enemy, and I believe that they have alterior motives whenever they touch a car.