Re: There's no way an app can be written to run on any Windows device
Err... They were not supposed to run on both Desktop and phone.
Microsoft's vision is that a tablet is a mini desktop screen, rather than an oversized phone. Thus you have Windows 8 running on tablets and desktops and Windows Phone 8 running on phones.
An iPad application is also different to an iPhone application. Just look at Facebook for the iPhone and for the iPad (I wish Android did this - Spotify and Facebook on my Infinity tablet are annoying as hell to use (to the extent that they are not used at all).
Whether it's better to treat a tablet as an oversized phone or an undersized desktop is open to discussion, but either way all companies know that there is a large difference between the desktop and the phone.
Maybe they will converge at some point, but no mater the OS it will rely on responsive design to get the interface flexible enough to work on all screen sizes (just like modern websites do...)