You are actually posting sales order stock into own stock at customer, quite similar to a 411 movement from sales order stock to own.
631 movement type is a WA type (goods issue movement) like 561 or 501 movements. Goods issues are valuated at material master price.
Because of this the value does not flow from sales order stock value to your own stock value, instead the sales order value is reduced at is calculated price, and the own stock is increased based on the price in material master at this time.
If you want the prices equal, then you need to adjust the material master price with MR21 before you do the transfer posting.