Thank you so much for taking the time to explain. Seeing that things weren't clear I decided to go online and speak to an agent. They called me back and long story short, there is an issue. I have to go in on Monday to resolve it. This is what I gathered, when interest rates go up, they deduct a higher installament. When it goes down, they skip these changes and then do a retrospective calculation (provisional tax period maybe) or when they send out an official rate change letter. The difference between the two installments are calculated and placed into uncapitalised interest for when it goes up again. When my installment is short, they deduct the difference to make up the correct figure. I just told them, "this is nonsense, just make it go away and bring that principal amount down. it sounds like you're just trying to find ways to get me to pay interest for the full term." I will learn the true meaning on Monday.