If this was for a tax year prior to 2018, which is likely at this point, the IRS is likely questioning why you reported a taxable amount less than the full amount of a pension distribution. This is usually the result of using the Simplified Method to calculate a taxable amount less than the full amount, particularly if box 2b Taxable amount of the Form 1099-R reporting the distributions is marked. It's not uncommon for people to use the Simplified Method when it is inappropriate to do so, or even if its appropriate (box 2b Taxable amount not determined on the Form 1099-R is marked), to enter incorrect values needed to calculate the taxable amount.
