Hatchbacks and aero...
It's possible for a coupe or a hatch to have a lower .cd than it's sister sedan but not necessarily.
Look up Kamm Back in wikipedia.
And/or go to http://ecomodder.com/forum/aerodynamics.html
and start reading the stickies at the top of the Aerodynamics forum.
Here's some of the how and why.
One goal is to make the air flow follow the car's contour. Or more accurately, design the contour so that the air can follow it. A curved or angled surface that the air can follow at 40 mph might no longer work at faster speeds.
Another goal is to create the smallest possible wake. Someone posted re. fish, birds, and may have also mentioned teardrops. Those are basically the ideal shape, with the tapered end of the teardrop at the rear. The air/water can follow the gentle curve/slope as it tapers, and the airflow over top, bottom and sides joins back together where the tail tapers together.
The opposite of a small tapered back would be a full-size square back, like an 18-wheeler trailer. It sucks a huge vacuum behind it. The vehicle's engine is working to produce the energy to create that vacuum. Wasted effort. It shows up as a higher .cd and reduced mpg.
You can chop off that teardrop tail at any point and that works very well. Not as well as continuing to a fine tip but better than a full width, full height, back end.
The benefit that a coupe has, is that the rear seats don't usually have the same headroom as the fronts. The roof can start sloping down a bit earlier so the angle is more gentle. That way the air flow can follow the body at a higher speed, or follow it better assuming the same speed. The Civic couple does this, in comparison to the 4-door. I visually compared the '96-00 body styles and it's true, sedan vs. coupe.
A hatchback can have a gently sloping rear that can also do this job. However, many hatches keep a lot of height, and the Civic hatch is an example. Not that good for aero, compared to a back that tapers to a smaller cross section, but handy for carrying stuff. And probably better aero than compared vs. SUVs and wagons, pickups, etc.