The web site http://www.w3.org/Protocols/rfc2616/rfc2616-sec10.html

shows the following

============================================================================
10.4.4 403 Forbidden
The server understood the request, but is refusing to fulfill it.
Authorization will not help and the request SHOULD NOT be repeated. If the
request method was not HEAD and the server wishes to make public why the
request has not been fulfilled, it SHOULD describe the reason for the
refusal in the entity. If the server does not wish to make this information
available to the client, the status code 404 (Not Found) can be used
instead.
============================================================================

As can be seen there is no hard and fast rule as to why this error header is
being sent to you :(

One reason I have found for this on some sites is that the http server does
not allow you to browse the server, preferring you instead to use the
navigation supplied in the web pages but as the above says, this is not the
only reason for seeing this.

Cheers,
Pep.