I recall seeing this issue on other forums. This is how BMW designed the car from what Ive read. If you look from the hatch or from the back all owners should notice it is offset somewhat. Many theories including safety in the event of a crash.
Here is one explanation from a different forum...
"I have looked at many many Z4's and they are all this way. Very irritating that BMW would build a car with a crooked wheel... I would be curious if there was a fix. The new Z4's do not exhibit this.
Many cars (and the majority of small cars) don't have the steering wheel perpendicular to the centreline of the seat. It is all a function of packaging, the relationship between the wheel-well, transmission tunnel and components in the engine compartment determine the alignment of the steering column shaft. In a small (narrow) car it is generally impossible to find a position that allows the steering column to be perfectly aligned with the centreline of the seat, especially if wide front tyres dictate a large wheel well for the size of the car
I've not noticed this in any car that is mid-sized and above, but have noticed it in many cars that are smaller than mid-sized."