Try the following until the issue is resolved:
- In the mailing, select "Edit mailing" from the left-hand navigation
- On the General tab, toggle the "Householding" setting and save the change
- Click "Calculate segment counts" to refresh segment totals
- repeat step 2 and 3 to revert the changes and refresh the totals
- re-run the "Mailing record counts" report
If the issue persists, an alternative way to obtain the same information in the report is to right-click anywhere in the segment grid and choose "Export Grid"