Stop and think about that question from a non-tech perspective. My mom (or yours) likely has no idea that there's a difference between "Windows Phone 8.1" and the "Nokia Lumia Icon with Windows Phone 8.1 installed". Now contrast that with Apple or even the Android world - they don't have such silly branding problems. No Apple user says "IOS 8? Is that the new iPhone?"
Branding is important and to brand the OS with the word "Phone" in it implies, to non-tech/news people, that it is a phone (which it isn't - it's just the OS). My mom doesn't want a "phone with Windows 8.1 on it" but she might want a "Windows phone". She doesn't want a "phone with IOS 8 on it" but she would like an iPhone.