An allowance of 1/16″ along each edge (or 1/8″ on the height and the width) makes it so that the other components can drop easily into the recess. For measuring purposes the overall sizes of the mat, glass and backing are all presumed to be the same size as the frame. But if they really were, it would make for a tight fit. So the frame is made slightly larger by adding an allowance. Nevertheless, we still presume all components are the same size. Think of it as a little lie we tell ourselves.