There could be multiple reasons. What year did you get the bill for? Was the $400 you paid to TurboTax or to the IRS or state? Who is the payee on the bank statement for the $400? Did the payment actually come out of your account or did you unintentionally select the mail payment option and inadvertently not send the payment?
The IRS could have randomly chosen your return to audit for the year you received the letter for, resulting in them catching an underpayment of estimated payments.
Did you owe a penalty for that year and allow the IRS to figure the penalty for you?
If TurboTax calculated a penalty and you paid it, then the IRS would not have a reason to notify you as you would be paid in full. The IRS would generally only notify you when there is a problem that they catch, or when the penalty amount is not included on form 2210
Those are a few of the reasons why you could have received the penalty letter from the IRS. Depending on your details, there could potentially be other reasons.
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