Those are generic reasons why they adjusted the Recovery Rebate Credit on your return. See this IRS article
https://www.irs.gov/newsroom/irs-letters-explain-why-some-2020-recovery-rebate-credits-are-different...
A common adjustment is if you claimed the Recovery Rebate Credit on 1040 line 30 when you already got the Stimulus payments or got the second one after you filed or reported the wrong amounts. Do you have an amount on line 30? And you had to report the right amount in each box. If you put the total in 1 box it would give you the other amount again. The IRS knows they already sent it to you. Line 30 is only if you qualify more.
There have been 3 Stimulus checks. You have to file or amend your 2020 and 2021 returns to claim them on 1040 line 30 as the Recovery Rebate Credit. But first double check your bank statements and check your IRS account (both accounts if married) to see what they sent you.
https://www.irs.gov/payments/your-online-account
EIP1 The first Stimulus payment was 1,200 for each spouse and 500 for each dependent under 17. Came out in spring of 2020.
EIP2 The second Stimulus was 600 per person and dependents under 17. The 2nd stimulus checks started going out at the very end of December 2020 and continued into January/Feb 2021.
The first 2 Stimulus payments went on your 2020 return even if you got the second one in Jan/Feb 2021.
EIP3 The 3rd Stimulus payment started going out in March 2021 and is for 1,400 for each person listed on your tax return, no age limit. The IRS used your 2020 (or 2019) return to send the checks out fast. If your 2021 return changes and you qualify for more you can claim the extra as the Recovery Rebate Credit on 1040 line 30. But line 30 is only if you didn't get the full amount or qualify for more. Not the amount you already got. The IRS also sent out letter 6475 to tell you how much they sent you . If you are married then each spouse should get a letter for half.
