Hi, I'm reading the man page for XF86Config to try to come up with a solution. I see in there an option called 'Virtual <xdim> <ydim>' that looks like it would be in the Screen Configuration section. Do you have any line in you're XF86Config-4 file that has 'Virtual' on it? If so it is probably what is causing your viurtual desktop to occur. It may be helpful if you were to post your /etc/X11/XF86Config-4 file for troubleshooting purposes.
HTH, Jim