That width:100%; is hard coded on the HTML, you can see this by checking the style attribute on the div tag in the HTML.
The easiest way for CSS and HTML debugging is by using the developer tools/web inspector tools (integrated in firefox and chrome or by plugin such as Firebug). The element.style CSS selector means it is hard coded on the selected element.
"hard coded", in this case, means it is directly injected through the HTML. Through the developer tools/web inspector tools you should be able to determine the origin of the CSS in question.