Here's my theory.
I think this was done to change people's ideas about what Windows is and where it can be used.
I think there was a danger that Joe Public (not someone with technical knowledge, like I imagine you have) would associate Windows with just the traditional desktop experience and assume it's not relevant as they start to buy other computers for home and personal use that aren't traditional PCs or laptops. (Think tablets, phablets, and phones but also smart TVs and connected devices plus whatever the future brings.)
By making it very obvious that it's the same OS on all devices running Windows 8. This reinforces the idea that Windows CAN run on multiple devices. It would be very dangerous for Microsoft if the public started thinking that Windows was JUST for PCs and when it comes to tablets (or anything else) you should look elsewhere (to Android or iOS).
Of course it's more complicated than that and involves many, many factors but I think this is an important consideration that others haven't been talking about. Yes - I do sometimes see my role here to be to remind developers that there are more than technical reasons behind technical decisions.