Wait until you get the letter from the IRS explaining how they calculated the additional refund. You should get the letter two or three weeks after you got the refund.
If the difference was exactly $1,200 you might have claimed a $1,200 Recovery Rebate Credit that you were not entitled to. That could happen if you told TurboTax that you did not receive a $1,200 stimulus payment, but the IRS did send the $1,200 payment to you. Is there $1,200 on line 30 of the Form 1040 that you filed? But you won't know for sure what happened until you get the IRS letter.
