WHY DO MULTIPLE PO'S CREATED FOR EACH BLANKET ORDER LINE WHEN WE CHANGE THE EXCHANGE RATE FOR THE CU

WHEN I TRY TO RELEASE  A PO FROM A BLANKET ORDER WITH MULTIPLE LINES EACH WITH DIFFERENT ITEM AND QUANTITY BUT SAME SUPPLIER AND BRANCH/PLANT, PO IS CREATED FOR EACH BLANKET ORDER LINE ONLY WHEN I CHANGE THE CURRENCY EXCHANGE RATE ON P0015 BEFORE I RELEASE THE ORDER.

I NEED TO HAVE ALL RELEASE LINES ON THE SAME PO.

DO ANYONE KNOWS WHY THIS HAPPENS, DO I NEED A SPECIFIC SETUP OR SOMTHING.