Typically "Authentic" numbers are heat pressed onto the jersey, then stitched. This is to keep the numbers in place when the sewing is done. There is no glue applied, the glue/adhesive is actually apart of the twill. So one side is the twill and the other side has a hardened adhesive on it that melts when heat pressed.
With the "Heat Pressed" numbers it is almost impossible to cleanly remove them from the jersey because they use a much stronger adhesive on the numbers. I suppose this is to compensate for the numbers not being stitched down onto the jersey. I've seen some heat pressed numbers removed and the glue residue left behind is very hardened and almost embedded onto the jersey.