I've often noticed that on the second turn of the inward leg the transports suddenly have picked up GS from somewhere, many times filling them completely. Since their Fleet Icon is still in the Atlantic Blockade Box I can simply cancel the rest of the inward leg of the trip and merge them back into the Atlantic Blockade Fleet to return them to supplying it.
Does anybody else see this happening

Is this WAD?
From where are they getting these supplies?