Collections (“directories”) have a trailing slash at the end, while non-collection members don’t. See RFC 4918 page 15:
“Wherever a server produces a URL referring to a collection, the server SHOULD include the trailing slash. In general, clients SHOULD use the trailing slash form of collection names. If clients do not use the trailing slash form the client needs to be prepared to see a redirect response.”
WebDAV/CalDAV/CardDAV are quite complex protocols and many servers return invalid responses (for instance, unencoded=invalid URLs that make parsing impossible) etc. So I suggest that if you implement these standards that you try your best to be compliant, at least for the important features.