Driving style mostly.
I've had days where i've gotten 10MPG....and then days on the highway where i've done 27MPG
Shifting a little sooner or taking your foot out of it a bit makes a huge difference.
As Brian said, torque is the key. Torque is what gets the car moving, not horsepower. The more torque an engine makes, the quicker it will come out of the hole in drags and the better the economy on the streets. This is why supercharged engines can be fuel efficient. The supercharger doesn't just add hp but a lot of torque as well. Take the SC version of the 3.8. The addition of the M90 added 70 hp over the N/A 3.8 but also added 110 ft/lbs of torque.