Check whether you experience the same issue in the theme's demo website
http://wp-themes.com/twentyeleven
You're right : there's not the same problem on http://wp-themes.com/twentyeleven... But maybe the theme version on this page is TwentyEleven 1.5.
I would like to test with a site having TwentyEleven 1.4, like on my website.
Do you know such a site ?