Probably because before they had cameras with the approximately the same MPs but in crop factor and FF (35mm) format. Obviously, you get more "pixel reach" with crop factor camera because you are laying more pixels on the subject. Now with D800 you get the same MP density as crop factor camera (not counting new 24mp crop camera) so my guess they are trying to show you what it would be in FOV instead of FL. Since FOV is what you see with your own eyes and FL is what you know maybe that is why it is easier to do it on FOV. Just a guess.