Many people had lower incomes during the pandemic in 2020 or they may have received unemployment benefits. These can affect the amount of any earned income credit and additional child tax credit you qualify for. The IRS will allow you to choose to use your 2019 or your 2020 income, whichever gives you the higher credits.
xmasbaby0 gives an excellent explanation, as well as the steps to use in the calculation, in the answer to a similar question: What’s the difference between using last year’s income or this year’s income for 2020 taxes
@April0914
