The weight of the shipment is always calculated as follows:
Empty box weight + Weight of all items in the return.
See here for more details on this https://docs.richcommerce.co/prepaid-labels/how-is-the-total-weight-of-a-return-calculated.
1) Box weight Usually, when you encounter a similar concern, it's prevalent that the set box weight hasn't been considered in the total weight (Return Settings >> Automatic Prepaid Labels >> Configure >> Box Weight).
Sometimes, items are shipped out in the initial outbound shipment with a box weight of 0 which is then exactly the delta in the weight and price difference.
2) Weight of items in the order (technical)
If you have a developer on your side, let them check the weight of the item on the order in question through Shopify's Order API endpoint (https://shopify.dev/docs/admin-api/rest/reference/orders/order). Shopify stores the weight of the item on the order itself as "grams". We use this data to then calculate the weight total as described above.
Note that changing the weight for a variant in Shopify does not change the weight on existing orders in Shopify.