The difference is: Microsoft brings Windows 10 to this device. Microsoft will not bring Windows 10 to the Pyra. Windows is not Open Source, so the needed changes can not be done.
Do you have insight into Microsoft's plan that others don't? Whilst there more than likely won't be a specific Pyra edition, you don't know what type of ARM releases they'll do. They seem to like their "Internet of Things" at the minute, so why couldn't they release a version that runs on OMAP5's and other TI boards - which coincidently would include the Pyra hardware (or at least some of it)? In fact, all they'd need is an ARMv7 build right? I'm not sure if it's quite that straight forward, but that's the same architecture as the Pyra.
Windows rusn on both Intel and AMD chips, imagine that (yes, I know they're "compatible"), it also seems to support dozens of motherboards, graphic cards etc.
I'm not saying Windows 10 will definately come, I'm not saying it'd work straight out of the box, I'm not even sure I'm arsed if it does, but unless you can categorically say that Microsoft 10 will not support the OMAP used in the Pyra, you have no idea that "it cannot be done".
Ugh, this hasn't helped my mood today.
Edit:
One last thing before I close the browser for today: Yes, you're right, it'll have to be Microsoft to bring Win10 to the Pyra (which is what I think you were trying to get at), but it could be that the Pyra is "supported" as a by-product whatever ARM releases Microsoft do - as opposed to being intentional