Why does a transaction drop out of rcv_supply when it is transferred but not delivered?

We have been using rcv_supply to identify when a Receive transaction has been fully delivered. If the items are delivered or corrected, it seems to work. But if there is a Transfer transaction, then the Receive drops out of rcv_supply even though the parts are still on the dock. Why is that? And what is a better way to identify when a Receive has been fully delivered? To be clear, this type of Transfer moves the parts from one receiving dock to another before inspection or delivery; it is different than transferring onhand parts.

Thanks in advance for your help,