Weight is different than sum of items

Hi all,

I have an outbound delivery order.

Every position has it's brut weight (double click on item, Tab "Addl Quantities").

If I sum these weights, I get 16.737,342 KG.

On header level, tab "Addl Quantities" has a weight of 16.875,714 KG

Checking the master data, on header level it's calculating with the weight of pieces, on item level system is calculating with cases.

How to tell the system it should use the sum of items?

Thanks

Michael