Heh nah, it's just speculation on my part. Though, it does kind of make sense.
With the Xbox 360's name, MS wanted to use a number that was higher than 3. They wanted it to make it sound better than the PS3, from a marketing perspective. I'm guessing they want to do the same here, and they already have Windows 8 in place. Plus, if they're going to promote cross-platform play between Windows 8 and Xbox 8 games, it does make sense to use a similar naming convention.
With the Xbox 360's name, MS wanted to use a number that was higher than 3. They wanted it to make it sound better than the PS3, from a marketing perspective. I'm guessing they want to do the same here, and they already have Windows 8 in place. Plus, if they're going to promote cross-platform play between Windows 8 and Xbox 8 games, it does make sense to use a similar naming convention.