Shipment quantities used to be pinned to whatever was originally planned. So once you packed, split shipments, moved quantities around, and made a few edits, the numbers drifted out of step with reality and told you to ship more or less than you actually owed.
Shipment quantities now calculate around the ordered quantity. Split a shipment and over-pack the first one, and the second shipment adjusts that line down on its own, so the total still adds up to what the customer ordered.