Shipment readiness used to check a line's demand against your entire on-hand pool, so several open orders could all show Ready against the same single unit. A shipper would pack one or two and the rest would quietly stop being ready. Available inventory is now split across competing open orders for the same item, with packed lines and earlier ship-by dates going first, so Ready reflects what is actually available for that specific order.
Rolling out over the next few weeks — if you don't see it yet, it's coming.