Why does the PL outstanding items (retrospective) report not equal the NL control account for the same cut off date?

An imbalance can be caused by documents from another period that have been transferred between different control accounts.

To identify transferred docs is to filter on the Journal Reference "Transfer" and source module "PL" in the NL transaction enquiry screen.

NL / Transaction Enquiry

All PL transactions that have been transferred will be listed, review and see if any affect the control account you are reconciling.  This is usually the cause of the imbalance.