Google glass is a HUD. When it is on, it also projects directly into your eye. My honours thesis was comaparing the angle between the eyes when viewing items in a 'virtual screen' (eyepiece HUD) and a real screen (half-dome field at 3m). Glass uses the same technology as the military HUD - throwing collimated light into your eye and using your retina as the screen. Not to mention that the glass is still in your field of vision when looking elsewhere, you're just not directly focusing on it. Your field of vision is 'everything you see', not 'what you're focusing on'.

Similarly, in its current incarnation, I can watch video on Glass. My radio and instruments can't do the same.