Theme Twenty Thirteen – white margin in small screens

Hi,

I have a new install of WP 3.6.1 (French version) using the Twenty Thirteen theme.
I encounter a problem that I have been unable to find a solution to since yesterday, very annoying yet it seems undocumented in Google / WP.

When I visit my site with Webkit browsers (Safari and Chrome), there is a ~20px wide white space (or margin) created at the right of the whole website. The problem does not appear in Firefox.

On desktop, this makes Chrome create a horizontal sidebar when the browser’s window is narrower than 1130px, everything is fine for larger windows.
On iPhone (Safari and Chrome), there is just a useless white margin on the right, which takes quite a lot of space on a small screen.

You can see it live at my website: http://www.architecte-maisons.fr/

Note that I haven’t made any change to WP or the template and that all extensions are off.

Thanks for your help!