Currently, Windows Phone and Windows Client are still separate development trees, which are slowing merging, they currently as of Phone 8.1 and Update 1 to client share 97% of the WinRT APIs and the same NT6.x kernel but there are still fundamental differences between the 2 trees. Windows Threshold (some believe will be called Windows 9, MS refers to it as vNext) is supposed to create the unified OS (in theory the third OS Tree Xbox will be brought into the fold as well).