As per Amigojack:
Outbox = waiting to be delivered (phpBB delivers the messages when the recipient logs in).
Sent = the message has been delivered (but not necessarily read)
What may be missing (for some) is a Read-box .... however, that's flirting with privacy-issues. I see no reason why someone should be notified that I have read the message.
A bug is a feature that hasn't made it to the manual (yet)