The retraction of the piston within the brake calliper is achieved by the piston seal. As the pedal is applied the piston protrudes slightly & the rubber seal flexes. As the pedal is released the seal re takes its original form & in so doing withdraws the piston. As far as I am aware that is all there is to it. Although I too am happy to be corrected.
Another point to consider is... How would a disc re-cutting tool be able to machine in this variable thickness?