Hi Michael,
basically the item weight/volume is usually (if no catch weight, ...) calculated based on the IM/ALTER UOM. So in your case based on cases. The result for an item is x KG.
The header just sums the list of the weight/volumes of the items . So the sum of the volume/weight of all items.
It does not sum up the number of cases of the items and then calculate the header weight. It just sums up the already calculated weights of the items (wich are in KG).
Best regards
Markus