order_shipment foreign keys question

[ronf]

Hi,

I have a question regarding foreign keys on the order_shipment table.  

If the primary key to order_shipment is composed of the PK's to order_item (order_id, order_item_seq_id) and shipment_item (shipment_id, shipment_item_seq_id) then why do the foreign keys point to order_header and shipment?

Ron Frank