DEF fluid levels won't affect mileage. It is post-treatment, after the combustion process.

The X5 is a bit of a brick. Pushing a brick too quickly through the air will reduce mileage. I'd put it down to that mainly, but ambient temperatures, fuel used, tire pressures, etc, can all have minor impacts as well.

__________________
2007 X3 3.0si, 6 MT, Premium, White

Retired:
2008 535i, 6 MT, M Sport, Premium, Space Grey
2003 X5 3.0 Steptronic, Premium, Titanium Silver
2002 325xi 5 MT, Steel Grey
2004 Z4 3.0 Premium, Sport, SMG, Maldives Blue