IRS knows all the income data only in simple cases like salaried employees with W-2 forms. If you are business owner or contractor the things can be more complicated and what IRS really knows is a guessing game. IRS is posturing and bluffing that they know everything and put pressure on a tax-payer to voluntary disclose as much as possible about their income out of fear of criminal persecution. Also they will rarely tell you that you "overpaid" your taxes unless it is a simple math mistake.