Line 30 of the Form 1040 is calculated based on entries the taxpayer has entered for the amounts received for EIP1 (Economic Impact Payment #1) and EIP2. If the amount entered as received differs from the amount of credit available, which is based on the filing status and number of dependents claimed on the return, the program calculates the recovery rebate credit which is the amount shown on Line 30 of the Form 1040.
An example of this would be: If you entered that you received $2100 for EIP1 but based on your filing status and the number of dependents listed on the return, your EIP1 should have been $2200, the program is calculating that you should have received an additional $100 which is the amount on Line 30.
The IRS has indicated that they may send returns that are claiming the recovery rebate credit through error resolution checks to verify that the credit claimed is correct. If the IRS finds that the credit was claimed incorrectly, they will adjust the refund.