Nothing - that' just the way css works.:)
You have specified 100% as the width of that element and 100% for that element will be whatever the viewport width is at the time.
You can't expect the element to be 100% wide and then suddenly be 150% wide because you want to scroll outside.
If you want the lightgreen to match the darkgreen's width the set a min-width (won't work in ie6 though).