Because you have a 1px border on each side of the #wrap div, it blows the width out to 792px.
My suggestion would be to reduce the width of .span-20 to 788px, and eliminate .navbase, instead adding top padding to the #wrap div and adding the .navbase background image to that div instead.
That’s the problem with frameworks and you must make sure your content fits exactly into the framework.
You shouldn’t really change parts of the framework to suit your design (unless you are 100% sure of what you are doing) because all the other rules depend on everything else being set up correctly.