The sale amount that you see in your Dashboard refers to the value of the sale that the retailer will pay on.
Some retailers do not pay on VAT or delivery charges (and some do not pay on gift cards or vouchers used as part of the purchase). This explains why the amount you see may be slightly lower than the original price of the item.
A friend has clicked through to an online retailer from your profile and spent a total of £100, £95 on a product and another £5 on delivery. If the retailer doesn't pay on VAT or delivery the amount you would see is £95 / 1.2 (VAT), giving a total 'amount' of £79.17.