I have a comprehenisve document describing the twin-axle suspension on the X5 (but only a paper copy), otherwise I posted an article on X5world that had a description of the various modes (although I'm not sure that fully explains your symptoms). If you PM me then I can email it to you as a Pdf file.

The only reason I can think why your rear would have been sitting visibly lower than the front, would be due to a ride height sensor providing a faulty reading, either that or a fault in your air pressure resevoir (so when the engine is switched off it doesn't have enough air to equalise the ride heights but when you turn the engine on, then the compressor comes into action and provides the necessary air pressure). I'd tend to err on the side of the ride height sensors (which are cheap parts) and have been known to be wrongly fitted at the factory. If they're not fitted at the horizontally or perhaps back to front (it has been known!) then that will cause it to perform unevenly. Probably a duff sensor which for a few dollars would be worth getting BMW to change. They can tell by taking a voltage reading across each sensor and then comparing the numbers, that should answer your questions pretty swiftly.

But just be clear that it's NOT normal...

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