possibilities are almost endless. a small additional refund would likely be because the IRS was tardy in making your refund and paid you interest (taxable when filing your 2021 return). if rather large I would check your tax return transcript using the IRS website. it will show amounts you reported and amounts per the IRS computers.https://www.irs.gov/individuals/get-transcript
If the additional refund is something other than interest, the IRS will send you a letter explaining what they changed on your tax return. You should get the letter two or three weeks after you got the refund.
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