Why do you want to “fix” this ? is it causing problems ?
“Unread” folder is just a search folder. Search Folders are “virtual” folders. They contain views of e-mail items matching a specific search criteria, in this case, mails that have not been read yet.
Mails in the “Unread” folder are stored in some other non-search folder, such as the “Inbox” folder. <b>So you should see them in the “unread” folder and in some other folder also.</b>
If you do not need the “Unread” folder, you can safely delete it (<b>just make sure that in fact it is a search folder, and not a normal folder the user created with that name</b>).