Depends on the server config. There are so many possible answers and configurations that you might need to check with your host. They could be restricting opens, you might be served from two different fileservers, so, while the path may look identical, you could be on two totally different paths, or many other possibilities.
You might do something like:
on each of your sites so that you can get the 'absolute' path. Though this might show you a chrooted path and not give you the true result. Your host should be able to help.