I think I'm starting to get the picture now. It sounds like the piston is a bit like a telescope which elongates as it unscrews. The bit it screws into (like a sleeve, although closed at the end where the handbrake cam is) is free to move in the bore, which is why you have to push and turn to retract the piston (to stop the sleeve rotating with it).
This means that the handbrake end of the telescope does not get much further from the cam as the pads wear, even though the other end is moving closer to the disk.
Thanks for the explanation.