the purple bar is the problem;
most elements in Twenty Eleven are flexible width, while the purple bar seems to be styled with s fixed width.
rezize your browser window to see for yourself.
remove the 'width' from this style:
#access {
background: #9966ff;
display: block;
float: left;
margin: 0 auto;
width: 947px;
}
ideally, move and merge the whole style with the original one a few lines earlier in style.css (shortend just to show it here):
#access {
background: #9966ff; /* Show a solid color for older browsers */
background: -moz-linear-gradient(#252525, #0a0a0a);
...;
width: 100%;
}
http://www.w3schools.com/css/
btw and important:
please don't edit Twenty Eleven directly, but create a child theme http://codex.wordpress.org/Child_Themes and make the edit in there.
this will also help to keep your customisations safe from getting overwritten with your next upgrade.