Weight is computed with unit_value and product variant_unit_scale

app/models/spree/line_item.rb

@@ -126,7 +126,7 @@ module Spree
     def copy_dimensions
       return unless variant
 
-      self.weight ||= variant.weight
+      self.weight ||= computed_weight_from_variant
       self.height ||= variant.height
       self.width ||= variant.width
       self.depth ||= variant.depth
@@ -236,6 +236,14 @@ module Spree
 
     private
 
+    def computed_weight_from_variant
+      if variant.product.variant_unit == "weight"
+        variant.unit_value / variant.product.variant_unit_scale
+      else
+        variant.weight
+      end
+    end
+
     def update_inventory
       return unless changed?