On some sight sets, they state that the sights will "fit all Glocks". Others are frame specific. Why is there a difference? On Amerigo's site, they list the front sight as a standard height for all Glocks. (.165) The rears are different by model, but I can't figure out why. Why would the rear sight need to be taller on a G20/21 than on a G17/22 if the front is the same height?