In Exchange you can encounter issues when you move mailbox between databases. And after this issues you can have, for example, 2 (or several) mailboxes in your databases which belongs to a single user. And of course, only one mailbox from them linked to Active Directory user account.
And one more thing - mailboxes that are not used, NOT marked as Disconnectedor SoftDeleted.
These mailbox are known as ghost-mailboxes.