I now check that your /friends page is actually a page. Some sites answer every URL with the same thing, so /friends looked real but was just a copy of their homepage. Now I also ask the site for a URL that can't possibly exist. If that comes back looking the same as /friends, then there's nothing there, and the submission doesn't get listed.
Nerdy detail: it compares the visible text of the two pages, not the raw HTML. Hashing the markup doesn't work, because one of the sites I caught this way hands back slightly different bytes on every single request. Strip the tags out and the two pages are word for word identical.