Tagged mails not all appearing at once, 'Local Folders' baffling me

I have three different POP3 accounts. When I set up Postbox it made a separate entry for my gmail account and the second two appeared as 'Local Folders.' I don't know what that means, or why the other accounts wouldn't appear under their respective descriptions.

Now, when I receive mail and tag it with my custom tag and click on that tag in the 'favorite topics' bar, not all of the tagged mails will appear. I have to click on the gmail and the 'local folders' accounts to see them all.

Surely clicking on the the tag should bring up ALL tagged mails regardless of where they;re stored. Can I just add that I still don't understand this 'local folders' concept.