Exchange Migrator 2.x
The 'Mailboxes to Migrate' report is reporting more mailboxes to migrate than what is recognized by the Exchange Administrator's 'Server Recipients' container for unhidden mailboxes.
The Mailboxes to Migrate report is based on the objects selected in a particular project. In this case, hidden mailboxes are being added to the migration project via Distribution List membership.
This is working as designed. Hidden mailboxes can be added to projects via a CSV import or by selecting the option Choose distribution lists and include their members.
To prevent hidden mailboxes from being added to a project you may select the option Choose mailboxes, custom recipients, and distribution lists (but not their members).