The glyphs are a result of both erosion of the stone surface (evident elsewhere in the temple) and the process of filling in and re-carving the stone to replace some of the original hieroglyphics. The technical term for such a surface that has been written on more than once is a palimpsest. The usurping and modifying of inscriptions was common in ancient Egypt throughout its history. The Abydos glyph was modified at least once in antiquity, and perhaps twice. Some of the filling has fallen out in places where the older and the newer inscriptions overlap, and the result is unique and odd-looking.
http://www.catchpenny.org/abydos.html