Could be the difference between adaptive modes on the gear box and also your aircon compressor being engaged. On the 3.0L the drag of the aircon can be noticeable. Try turn the aircon off, and see if the X pulls harder, also at the same time try it in sport (unless you have manual transmission?), which will force the most aggressive gear box mode. If this does not make the difference that you are noticing then I am fresh out of ideas....

__________________
X5 2006 3.0d M57N
GA6HP26Z Tranny
180,000km