Fit-screen dosn't fill all the screen
Fit screen &amp; full screen are not the same thing. 

Fit screen makes a 'guess' based on window content &amp; probable usage [per app, it would appear to not be consistent, some apps are better than others at 'guessing'] 

Full screen is under system control &amp; will generate a new Space for the app, filling the entire screen.

Clicking the green dot, top left of a window, will set Full-screen.
Alt [opt] clicking the same dot will set to fit-screen.