Many reasons.

Throttle calibration
Valvetronic
6-speed automatic
Gearing- it's taller.
slightly better aerodynamics (0,34Cd versus 0,35Cd I believe)
Reduced rolling resistance in tire choices.

International models:
Active aerodynamics
Brake Regeneration

Remember though: It's not what you drive, but how you drive. I have a long-term average of 8,9l/100km in my X3 despite many cold starts and short trips in a mountainous region.