There could be different reasons you did not get a full refund when you return an order.
You may have been charged a Restocking Fee. To locate the Sellers Return Policy, you can click on Seller Info and it will explain restocking fees in their return policy.
Another reason that this may occur is what is called a Short Credit.
Short Credit is when you earned Rakuten Points on an order. You then spend the Rakuten Points on another order. You then decide to return the order that you earned the Rakuten Points on. Since you spent those points already, we need to deduct that amount from your refund.