When running the Inventory | Mailbox Growth Report, the number of mailboxes returned is accurate but does not match the Inventory | Mailboxes | Primary Mailboxes report. In MessageStats versions prior to 6.10, the report not matching the Mailbox Growth Report is Inventory | Mailboxes
E.g. Inventory Primary Mailboxes report returns less records (mailboxes) then the amount of records returned in Mailboxes Primary Growth Report.
Allow hidden mailboxes in Mailbox reports is not selected in the Tasks node | Delivery Time threshold tab of the MessageStats console
1. Open the MessageStats Console
2. Right-click the Tasks node and select Properties
3. Click the Delivery Time Threshold tab
4. Ensure "Allow hidden mailboxes in Mailboxes Reports" is selected
5. Click OK
6. Rerun the Mailbox growth and Mailboxes reports.
The amount of records should now match up.