If you find that the product weight is not importing correctly from Amazon, this is usually because your product is also listed by other Amazon sellers. If the weight imported from Amazon for that product is different than what you enetered on your listing, this usually happens because Amazon is using an average weight between all sellers of the product.
In order to override this value from Amazon, you can simply use a product default.
For example, if you are importing your product from Amazon by their SKU, you will need to go to your Products page, find this SKU, then open up the product and set a product weight. This will then override any weight that is imported from Amazon, for that SKU.