Earlier quoted context omitted.
I believe it's a continuation-style server—hence fnid: function id—and the continuations are only kept around in memory for ~5 minutes.
I never understood why the continuation couldn't instead be addressed by a URL path. It could even get constructed from URL/query data if it moves out of memory, so keeping them in memory would only be a caching mechanism.
[addendum] I Reread your comment and realized that wasn't what you meant at all. What would be the benefit of path based uris over query string params in hn's case? I only see how they be equal, not better.