There is also the possibility that the battles are ending in ties. In that case, no resources are won at all. Even without cargoes, a small amount of resources should return unless all cargo space is used for fuel.
Fuel is burned at launch. Even if all the space was used on fuel at the beginning of the mission, then it would become available to take the defenders resources on the attack anyways.
Death begins with life's first breath, and life begins at the touch of death.