if you're just looking at the Fed tax due monitor that could encompass other things happening as a result of inputting more income and increasing your AGI - deductions could be phasing out, you may be paying Net Investment Income Tax (3.8%), it could be affecting the rate on Long Term Cap Gains / Qualified Dividends etc; also that total includes any penalties due based on what has been input so far. To understand properly how a 1099-INT is affecting your tax you need to look at what is happening to the different lines on the Form 1040 and a key worksheet in the tax calc is "Qualified Dividends and Capital Gain Tax Worksheet".
