At the point of booting the BIOS will typically throw out a 4:3 ratio image of 800x600-1024x768 resolution, also at the point of booting the GPU drivers will not have been initiated, as such the BIOS will tend to fill the screen, stretching to your displays aspect ratio.
Some monitors have the ability to set the display mode between...
dot-to-dot (pixel match, would draw the incoming signal pixel perfect without any stretching in the center of the display).
keep aspect ratio (will try to fill the screen keeping the original aspect ratio by adding black bars to the sides).
full (will fill the display, stretching the image and disregarding any aspect ratio).
Take a look through the options menu for your monitor to see if it has any built in scaling methods, unfortunately the manual for this display appears to be elusive so I cannot provide a 100% answer, but rather advise what to look out for.
If your monitor fails to support these scaling methods then I am afraid there is little that can be done to resolve this at a BIOS level.
Note the selected scaling method would apply to all incoming signals, including windows, although if you have your resolution set to that of the monitor in windows then this should not be an issue.
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