You can checkout the mobile layout of commercecanvas on your iphone to see the issue. http://commercecanvas.com/
Even though none of the widths are more than 300px, it allow you to horizontal scroll, and there is a white line of about 20px down the side. Very annoying. Not sure what to check to stop it from happening.
It doesn't do it in landscape mode, so it's like somewhere a width on one of the main elements in the mobile layout is being set to a hard width of 400px or something.