why would an exchange mailbox not have the "mailbox - username" item in the inforstores collection?

Hello,
 I am using Ruby w/ Win32OLE to access exchange mailboxes. Everything works great - however, there is one particular user that I can't access. For my program to work, I have to gain access to his Mailbox item in the infostores collection:

 IE:, @available_folders = session.Infostores
      @available_folders.item("Mailbox - username")

This works for every user except this one guy. I investigated a bit further and revealed that his infostores collection consists of two folders "Public Folders" and "Private Folders" whereas every other user in Exchange has "Public Folders" and "Mailbox - username".

Is there a particular reason as to why this would happen? Else, is there a way I can fix this?

Thanks in advance,
 - Jeff Miller