I get a full screen on my PC, on my laptop screen 9 doesn't use the full screen for this reason.
The laptop has an LCD screen, with a set number of pixils,
Windows will magnify screen 9 , but only by an integer, if it has enough room it will make one screen nine pixil either 1x1 or 2x2 or 3x3, it uses the highest one possible, but becuase the resolution is a weird one, it will never take upt he full screen.