Your desktop resolution is larger than your monitor resolution. When it detects your monitor, it lists available resolutions (modes) for your monitor. A few things to consider:
  • Are you specifying a resolution? e.g. 'screen=1024x768'
    If so, are you specifying one that is listed for your monitor? e.g. specifying a widescreen resolution that is not listed for your monitor in the detection.
    You may have to specify a larger/smaller resolution than you are currently.
    You may have to specify a resolution if you are not.