Thanks, @musicmaster!
Some additional information: It's with regard to variants/combinations. My main product has a weight of 0 and the variant has an additional weight of 1kg per unit.
I checked the tables (albeit after having corrected the weight in the DHL module) and I think we've found the culprit:
tb_product: It's correct, 0 for the main product.
tb_product_attribute: It's correct, 1kg.
tb_cart_product: Mine doesn't have weight.
tb_order_detail: It's correct, 10kg.
tb_orders: 139! (total_products_wt)
Where does the total_products_wt come from?