Try the following until the issue is resolved:

  1. In the mailing, select "Edit mailing" from the left-hand navigation
  2. On the General tab, toggle the "Householding" setting and save the change
  3. Click "Calculate segment counts" to refresh segment totals
  4. repeat step 2 and 3 to revert the changes and refresh the totals
  5. 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"