True from Microsoft's perspective but from a customer's perspective it looks like "Windows for phones" changes radically and entirely incompatibly pretty often.
Edit: On the other hand, most of the current market probably didn't know about Windows Mobile so perhaps that's mostly moot.
We could just as easily say "twice in 2 years, or once every year" since the first WP7 devices shipped in late 2010 and WP8 will be out by end of 2012.
Windows Mobile was substantially different and went through many generations with upgrades dependent upon carrier implementation.