An IRS audit isn’t because you underpaid the amount they think you owe — for that they usually send you a form saying, in their own special language, “hey I think you may not have included this XYZ income in your tax return, we think you owe an extra $ABC”.

Instead, it’s because they want to make sure you were truthful in what you reported, and may have some reason to believe you were not. Often, they don’t know how much you owe in this case.