Why does Page Speed return different results from those reported by Site Performance in Webmaster Tools?
Site Performance shows Page Speed suggestions based on content served to Googlebot (as opposed to a regular user’s browser). You may have rules defined in robots.txt or your web server configuration files that serve content differently to Googlebot. If so, you should try to eliminate such differences, so that you get consistent Page Speed suggestions everywhere. See also the Site Performance Webmaster Central page for more information.