Good question...I would like to know the answer to this too.
What I think is that the piston will adjust the pads to the correct position to the rotors when you pump on the brakes a few times after you are finished. The brake fluid will aid in this job and to keep it there at that distance. There might be a spring that plays a role in this too?
Anyone...please correct me if I'm wrong...hehe
'03 325i...Just getting started!