Turbo Tax is a self prepared return. They only guarantee the calculations are right based on your entries. They won't know if you entered something wrong or twice or in the wrong place or left something out. It doesn't check against any outside sources. The IRS has copies of all the W2s and 1099s your employer and banks/brokers, etc. sent in to match against. Turbo Tax only checks the calculations of what you entered. It is your responsibility to check it over before you file.
What is the 100% Accurate Calculation Guarantee
https://ttlc.intuit.com/community/charges-and-fees/help/what-is-the-turbotax-100-accurate-calculatio...
The difference sounds like the 3rd Stimulus payment. Did you get the 1400 or 2800 Stimulus check?
Check 1040 line 30. A common reason this year is if you tried to claim the Recovery Rebate Credit (for the 3rd Stimulus 1,400 per person) when the IRS already sent it to you. Line 30 is only if you didn't get the full amount or qualify for more. Not the amount you already got.
