I copied your code and, using an inline style, edited the div widths from "100%" to "99%" and the scrollbar disappeared.
I'm thinking this might be indicative of some padding that's adding to the overall width of the divs. For example,
Thus making the overall width .05em + 100% + .05em and too wide to display without a scrollbar. Check your padding (and margins too) to find out where you've gone awry.
As a hackneyed alternative, a quick fix might be to set the div widths to 99% then use a background image applied to the body tag and have it only repeat horizontally. This will prevent the horizontal scrollbar but still make your menu look as though it spans the width of the page, if that's what you want.