The difference is $2100 so it this the amount on line 30 of your form 1040? Did you & your spouse file a joint 2020 return with one dependent on it ? If you did then you got $4200 in the third stimulus payment sometime from March to December of 2021 ($1400 x 3 on the return) and I bet you only reported getting $2100 and not the full $4200 you got ... did you only get one 6475 letter from the IRS for one of you showing only $2100 ? Even if you did not get the second letter for the other spouse for $2100 you still had to report it ... you were only allowed the credit OR the advance payment. So the IRS says they sent you the full $4200 and you are not entitled to the $2100 credit.
