If you want to manage a user's attachments and go there in the ACP the pagination won't display every attachment of that user.
Posts per page: 6
Topics per page: 18
On the first page there are 6 attachments displayed.
You got to the next page and 6 attachments are displayed again, but the start parameter shows you that the start isn't at 6 but at 18!
So per page 12 attachments are not displayed to the user.