IRS calculates tax penalty independently of TT, they use the same calculation but if there is any adjustment/difference in the 2023/4 tax information or estimated tax payments in their system vs. what is input into TT that may give a different result. See lines 1-9 of Form 2210 for the safe harbor calculation, based on the smaller of 100% of your 2023 tax (110% if AGI > 150k), or 90% of your 2024 tax. If you made estimated tax payments you can check the payments IRS has on record under your account at irs.gov.
