Web Site:  100 % Screen Fill Monitor Resolution:  800 pixels x 600 pixels Web Site:  100 % Screen Fill Monitor Resolution:  1024 pixels x 768 pixels         Web Site:  100 % Screen Fill Monitor Resolution:  1152 pixels x 864 pixels Web Site:  100 % Screen Fill  (Wide Screen) Monitor Resolution:  1440 pixels x 900 pixels

All of the screen shots above were taken of the same web site at the same time, and yet you can see how the monitor resolution determines the lay-out.

So, many Web Developers create their sites to a specific pixel width, and leave the back-ground to be determined by the monitor size and resolution.   The InterNIC standard is 800 pixels x 600 pixels.......as it has been since the mid 1990s.  Though monitors in the US are now shipped at a resolution of 1024 x 768 or higher, this is not the case all over the world.  So, many Web Developers create sites of approximately 770 pixels wide to accommodate 800 pixel wide monitors without forcing the viewer to scroll left-to-right:

Web Site Built To Be 770 Pixels Wide:

Web Site:  770 Pixels Wide Monitor Resolution:  800 pixels x 600 pixels Web Site:  770 Pixels Wide Monitor Resolution:  1024 pixels x 768 pixels         Web Site:  770 Pixels Wide Monitor Resolution:  1152 pixels x 864 pixels Web Site:  770 Pixels Wide  (Wide Screen) Monitor Resolution:  1440 pixels x 900 pixels

To accommodate American viewers, some Web Developers are now building sites to 1000 pixels wide.  This looks better at the higher resolutions, but those with older monitors must scroll left-to-right to see the whole site:

Web Site Built To Be 1000 Pixels Wide:

Web Site:  1000 Pixels Wide Monitor Resolution:  800 pixels x 600 pixels Viewer must scroll left-to-right to view page.	Web Site:  1000 Pixels Wide Monitor Resolution:  1024 pixels x 768 pixels Site appears to "fill"  the monitor screen.
Web Site:  1000 Pixels Wide Monitor Resolution:  1152 pixels x 864 pixels Appears to have small border around site.	Web Site:  1000 Pixels Wide  (Wide Screen) Monitor Resolution:  1440 pixels x 900 pixels Appears to have larger border around site.

So, if you're looking at a site that doesn't fill your entire screen, it's probably because the site was built to a specific size, and that's NOT the way YOUR monitor is set.