Why is Turbotax using 2020 earnings instead of 2019 to recover missing second stimulus payment?

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The payments were an advance credit for 2020.  2019 information was used in sending out the payments since 2020 tax returns had not been filed at the point the stimulus payments were sent out.  They are being reconciled when you do your 2020 return.  In some cases, people may have been given a stimulus payment based on their 2019 income that did not qualify based on their 2020 income.  In these cases they are not required to return the stimulus, however they are ineligible for the Recovery Rebate Credit.