Part way thought the model life, the manufacturers often make a few changes to either improve the car or improve sales - usually both.
In late 2003, BMW started building facelift E53 models. New headlights and taillights, grills, bumpers, engine updates, new XDrive system and a few other things.
Have a read of this...
http://www.bimmerforums.com/forum/sh...ges&p=21403324