Unfortunately, it is probably correct. If you look at the actual tax table in the instructions for form 1040, such as on page 68 of this PDF, you will see that this can happen.
https://www.irs.gov/pub/irs-pdf/i1040gi.pdf
The tax table goes in increments of $50, so that if you are single and your taxable income is anywhere between $20,000 and $20,050, your tax will be $2206. As soon as your income tips over $20,050.01, your tax becomes $2212 and remains $2212 until your income hits $20,100. Without the interest income, your taxable income must be right at the very top of one of those $50 increments on the tax table, so that the interest pushes it over into the next level of the table.
Unfortunately, TurboTax can’t calculate an exact tax based on percentages because it must use the forms and instructions approved by the IRS which include the tax table.
