In The Aria software each type 0 mailbox can be allowed to have a home and 2 guest mailboxes. They are assigned a separate password so a user can receive messages from the mailbox owner and they can send messages to the mailbox owner only.
In some cases the mailbox owner can send messages to a home or guest and not realize that they are doing that, it is easy to setup a distribution list in your mailbox and add a home or guest address and then when you use the list the message is in essence sent to your own mailbox but you do not know this as you would only hear the message if you login to the home or guest 1 or 2 mailbox.
The best way to see if this is happing is to run a message aging report in menu 14 (Reports) if the mailbox shows 25 messages but when you log into it it tells you you only have 1 or any number less than 25 then the messages must be in a home or guest mailbox.
It can also be caused by future delivery messages meaning that you can fill up a mailbox with future delivery messages and you do not know they are there as the delivery time has not been reached yet, they will show on the message aging report as future delivery messages.