Possible reasons -
If you do not have a page named index.??? in your domain directory, the server re-directs to a default page. It is a security issue to prevent anyone from getting access to your directory structure.
Your site will only auto-run if you have an index.??? page in the domain directory.
Ditto, if your index.??? page is in a sub-directory.
If this is the case, the only way to load the page is to specify the full URL and file name, which the marker is unlikely to do.
It has nothing do with any specific browser.
[Edit - It is of course "highly unlikely" that you could get an A if the marker could not access your site.]