Virtual hosting is a method for hosting multiple domain names (with separate handling of each name) on a single server (or pool of servers). This allows one server to share its resources, such as memory and processor cycles, without requiring all services provided to use the same host name.
This web server is responding differently when the Host header is manipulated and various common virtual hosts are tested. This could indicate there is a Virtual Host present.
Consult the virtual host configuration and check if this virtual host should be publicly accessible.