The cause of the discrepancy is due to the way in which the report dates are specified. The dates entered in the Reviewer Activity Detail and Evidence of Review reports are based on Capture date, not mail date or Archived data. As such, the Date facet filter in the Review tab, is not the correct date facet. The correct date is the Capture date facet.
To resolve this issue and ensure the accurate count of reviewed items, be sure to select the Capture date Facet Filter in the Review tab and then compare the counts with those listed in the Reports