Interesting.
Funny enough, my previous X6 (not an M) was a demo and had some miles on it, I don't recall the exact number. Never burned any oil. At least not enough to where I had to add any in between regular service intervals. Although it had the N54 engine which I am not sure is known for burning oil or not. I don't doubt it was driven hard with no regard to break-in. I didn't pay as much attention with that car, it was a lease which I didn't plan on keeping.
The X6M is my baby so I mostly followed the break-in. She was a keeper from day 1
I have broken in several brand new Ducati's and followed the recommended break-in guidelines, similar in practice to what I did with the X6M, and they've done extremely well.
How many miles do you have on your X6 now?
Note also I tend to drive mine pretty hard somewhat often. Let's say she has made good friends with the governor and has met him on many, many occasions
That may be a contributing factor, no?