IMO it's simple:
XBox = SUCCESS
WP7 = FAIL
Why limit the XBox to the WP7 failure? I know this goes against the grain of their other direction around moving development to the W8/WinRT/Metro lock-in but as a descision in and of itself IMO it makes sense.
Makes me think that if the rumors of SL being ported to the XBox are true maybe Plan B for Windows is if W8/WinRT/Metro is a fail too that they can always fall back to SL everywhere. Now I like that idea -- as Plan A of course.