The Recovery Rebate Credit is calculated based on your 2020 income, whereas the first and second stimulus payments were based on your 2019 tax return (or last return filed). This was decided and signed into law by the Coronavirus Aid, Relief, and Economic Security (CARES) Act.
Generally, if a taxpayers' situation changes in 2020, they could get a partial or the full stimulus payments based on their 2020 income, if they did not qualify previously. However, if a taxpayer got the full amount for the first and second stimulus payments, they would not qualify for the credit on their 2020 tax return.
What is a Recovery Rebate Credit?