I too agree with the OP (although indeed, it's not the end of the world, it's easy enough to explain to new users what happens - it is a recurring question, though). I don't really care what other board software does, as long as this board software works intuitively
In this case, it's down to terminology. I understand why the email-terms are used, but the source of the confusion is that the various stages a PM goes through are just not equivalent to the stages an email goes through when in the same "mail box".
In case of an email, a message enters the outbox before leaving the user's system (or possibly their email server - what matters is that the reciepient at that point is completely unaware of the message). A user can understand intuitively when an email moves from their outbox to their sent box; either because they configured it to do so automatically, or because they take explicit action. In case of an email, "sent" means just that; the email was sent away. It doesn't say anything about what happened to the email on the other end (there might even have been a problem delivering the mail).
In contrast, in phpBB's PM system (which, despite parallels in usage, works very differently from email, of course), the sender has no control over when the message moves from the outbox to the sent box, it is the recipient that controls when the message moves. Additionally, when a message is in the sender's outbox, for all intents and purposes (i.e. from the users' point of view) it already is in the recipient's inbox, ready to be read.
In other words, a message in the sender's outbox is "sent" as well, because the recipient is aware of it. Essentially, what are now called "outbox" and "sent" box, are two different flavours of a sent box; one with messages that have not been opened yet by the recipient and can still be edited by the sender, and the other with messages that have
been opened and can therefore not be edited any longer to avoid confusion.
I am not sure what would be the best terminology, maybe it would even be best to merge the outbox and the sent box into one screen and just flag messages, but I do agree that the current behaviour is unintuitive.