On the rear brakes, the piston has a " step " on the face.
If the piston was turned while retracting it, the angle of this " step " has to be set, to get the correct ( square ) contact patch on the pads.
Have a very close look at the picture & you will see the " step ".
http://img.eautopartscatalog.com/live/N104018100ATE.JPG