[systemtest]

Perhaps it's a country thing. Here in the Netherlands that is how the banks calculate the prepayment fine. You are allowed to pay off 10% to 20% of the original sum per year but over that amount you pay a "boeterente" (interest-fine). It's equal to the amount of money the bank loses by loaning your money to someone else over the remainder of the fixed period (10/20/30 years).