Perhaps your tax return underreported taxable Social Security income. Many people this year mistakenly answered Yes to TurboTax's question that asks if they were a US citizen who lived abroad in certain listed countries, causing TurboTax to show on Form 1040 line 6b that none of their Social Security income was taxable. If you receive Social Security income, at your AGI level I would expect the amount on Form 1040 line 6b to be 85% of the amount on line 6a.
