Onhand gets updated from the dropshipment PO quantity

Hi,

We have done the necessary setups for a drop shipment scenario and manage to performed the drop shipment (Initial on hand quantity for the item was 10 Qty and PO has created for 50 Qty from the drop shipment).

Now the order lines are in "Close" status.

After the drop shipment , When we check the on hand for the item , system has updated the on hand quantity as 60 Qty and Available to reserve as 10 Qty.

1) Since we do not have the PO quantity of 50 Qty physically , what is the logic behind updating the on hand quantity by the qty mentioned in the drop shipment PO?