Because there's basically no difference between logging in and viewing as 'a guest'? The only ting that's noticeable when I don't log in is that 'hey you like football? Register now and see 50% less ads etc etc etc' pop up down at the bottom of the page.
Xenforo 2 default settings also don't differentiate between bots/search spiders & real guests, meaning the real number of guests is considerably less than the total shown.