There is a difference b/t WL and rejection -- at least in theory. A WL letter means that, if there aren't sufficient acceptances to NASS, you have a shot at getting a spot. Just as with the appointment WL, it's a numbers game. A rejection means you weren't accepted.
However, please understand that not being accepted to NASS doesn't mean you don't have a shot at USNA. The converse is also true.