Not really a 'problem' - AFAIK, C&D testing is somewhat unique in that:
a) They conduct instrumented testing of most cars (not simply regurgutating manufacturer performance numbers a la Motor Trend)
b) They correct their numbers for atmospheric conditions (i.e. temp)
c) They report exactly what they find: If they get a brand new (i.e. slower, not broken in) car, they report whatever performance numbers are actually recorded, and make no excuses. Production variances from one car to another DO happen. So theoretically, a even through a 330i is lighter and suffers less power loss through the added drivetrain components of the xi, maybe they got a particularly good example of an xi, hence the lower 0-60 times.
d) Like that other post says - the additional traction of AWD could also be a factor.
This points up the key issues in terms of all testing though: a) There are a million variables, and b) Any car is only as fast as it's driver.