I filed 2020 return "accurately" using all information available to me, using TurboTax (windows desktop). After receiving my refund, long after the 1099 deadline, I received a 1099-NEC from Postmates. I used TurboTax to generate an amended return. TurboTax told me I owed $202, which I paid.
I've now finally received a letter from IRS indicating that I should have paid $212. They want the $10, plus interest.
Pulling my Record of Account from the IRS website, I find code 767, Reduced or removed credit to your account of $65, and code 765, Reduced or removed earned income credit $147. These together add u to the $212. My amended return shows $137 reduction in EIC, so that's where the $10 discrepancy comes from.
Is there any way to determine why TurboTax calculated an EIC adjustment different from what the IRS calculated?
Not with this amount of information there isn't.
It is possible that the IRS is wrong and the EIC should have been reduced $137 instead of $147. In fact that seems most likely. But the argument with the IRS isn't really worth ten bucks and they know it.