Because you don't have a dependent on your tax return, TurboTax figures the 3rd stimulus payment is for two people, you and your spouse. That is why you see the calculated amount is $2,800.
If the $1,400 you received on your deceased mother's behalf is erroneous, you should consider sending it back to the government.
Follow this link to the IRS website to learn how to return the 3rd stimulus check.
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