remember that when accessing the physical X display on the server machine, you have to modify the settings in the control panel of the server. Changing the X server resolution of the physical display (f.e. :0) normally requires changing the Xorg.conf file, selecting the graphic driver, selecting the applicable acceleration methods. This is under responsibility of the Linux distribution or the configuration software provided with the OS. Virtual desktop created by NoMachine are under our exclusive control and thus don’t have similar limitations.
See also this: