Here is a little explaination from the Phil Knox from the Toyota Tundra discussion.
Your hatchback has a station wagon type wake of turbulent air, however, it is free of attached vortices.The sedans wake actually begins at the end of the roof as flow which cannot follow the steep backlight angle separates. Also, while this flow may reattach at the end of the trunklid where the main wake begins, the radical pressure gradients and violent mixing zones caused by velocity discrepancies between dide flow and rooftop flow can cause attached vortices which can trail the car by hundreds of feet,robbing horsepower from the engine.
GasSaver: 2000 Honda Insight Silverstone
Track Terror: 2002 Honda S2000 Gran Prix White- lots of mods - 28mpg
Beater: 1988 Honda Civic DX Hatback - Stripped - 30mpg
RIP: 1996 Honda Civic LX 42mpg - you will be missed