The last delivery indicator needs to be manually updated and as a consequence the PO's "No further confirmation" will also get updated.
if any of these two are selected, the PO will be closed for further confirmation.
i am not sure if these two checked boxes gets automatically checked (i assume thats your expectation if the delivery is completed)...
The reason is that we can always change the PO to increase the quantity or duration and allow further confirmation after the change.
I hope that explains your question.
regards,
Tashi