Why can't images/text be displayed on the LCD and the external display unit simultaneously using the SXGA+ (1400 x 1050) resolution?

Images/text cannot be displayed on the LCD and the external display unit simultaneously using the SXGA+ (1400 x 1050) resolution.

To display images/text on the LCD and the external display unit simultaneously, please note the information below:

For Liquid Crystal Display (LCD) -
Screen resolution: 1024 x 768
Colors: Maximum (32 bits)

For external display unit -
Screen resolution: 1280 x 1024
Colors: Medium (16 bits)

Images/text will be displayed only in the external display unit if it is set at the same resolution as the LCD. If both screens are used at the same time, the external display becomes a virtual screen (panning mode).