When resized to 800x600 border does not span 100%

[deronsizemore]

www.deronsizemore.com/test

Just wondering if anyone can explain why when you resize this page to 800x600, the border top, and the background color of "secondary" do not span 100% width?

I guess it's not a huge deal as I'm not doing this design to cater to 800x600 particularly anyway, but just curious.

[Cutter]

background color resizing? I don't understand what you mean.

[deronsizemore]

Well if you view the site at 1024 width it's fine, but if you view it at 800 using Firefox's web developer resize tool the blue top border and the blue "secondary" area are not 100% width anymore.