There are a couple of reasons that your interest will vary each month. One is that it is calculated on the daily balance of the account. Each month you pay off a portion of the principal, therefore the repayments on the next month will be a little different. Secondly, with a variable rate, the interest rate fluctuates with the standard cash rate, meaning your interest will fluctuate with it. Finally, your repayment will be calculated on how many days were in that month, so some months will be higher than others.