Further information about this matter:

A PO History screen shows what appears to be an adjustment against the PO in question. 

How does one print a detailed report showing the adjustment of the quantity?

Invoice Adjustments can cause the PO status to be set to completed like above. 

How would this adjustment have been done in PO since there are no Returns or Debit/Credit Notes transactions done for this supplier against the PO or Receipts?

Check the IC Inventory Movement report for the item. It should show a Receipt Adjustment

Sorry. I should have mentioned that this is a Non-Stock Inventory Item.

That makes it a bit trickier. Compare the PO Analytical Transaction List between Receipts & Invoices. It show the difference. Fact is, if an invoice is captured with the outstanding qty, even though, less were receipted, the system will recognize it and adjust accordingly and mark the PO as completed. 

That makes it a bit trickier. Compare the PO Analytical Transaction List between Receipts & Invoices. It show the difference. Fact is, if an invoice is captured with the outstanding qty, even though, less were receipted, the system will recognize it and adjust accordingly and mark the PO as completed.