There are basically two different heights of back-up flip up sights.  They are designed to fit gas blocks or on quad rails.  Since most gas blocks sit below the optics rail on an AR15 receiver, they are taller to compensate for the lower mounted rail on the gas block.  Front sights are designed to fit on a hand guard quad rail.  The quad rails are usually at the same height as the receiver optics rail.  The flip-up front sights are shorter in height vs the ones designed to fit on gas blocks.   They are not designed to be mount on quad rails, as the front sight post will be too tall.