Two possibilities. First, if your income was over $150,000 (or $75,000 if married filing separately), the safe harbor is 110% of the prior year tax, not 100%. Second, if you are paying some of your liability with estimated taxes, the calculation is done on a quarterly basis (withholding is assumed to be paid equally throughout the year). If you didn't pay enough in early quarters, paying everything later will not cure the underpayment deficit from earlier quarters.
If your income is not received roughly evenly throughout the year, you can see if you can avoid an underpayment penalty by using the annualized income method, Form 2200AI. Or you can have the IRS calculate the penalty if you don't believe the TurboTax calculations.
