Blanket Purchase Order authorisation

I am trying to understand and apply the Blanket Purchase Order system in Navision 5.0. We are using regular Purchase Orders and document approvals. However when I create a Blanket PO and approve and release the order, then create a regular PO from the Blanket PO the regular PO status is open and not released. If the Blanket PO has been approved and released, why does the regular PO it creates require a second approval and release?

Any help is much appreciated.

yes this is standard navision … because you may able update Qty to recieve in PO … or update anything new for PO … only release and continue normally.