In some cases, TurboTax started your return with the amount of the stimulus payments added to your refund. Then when you went through TurboTax, it asked about whether you had received the stimulus payments. If you already had received your payments and then answered had received both stimulus payments, then Line 30 changed to zero, because you were not owed a back payment.
In your case, the IRS found that you had already received your two previous stimulus payments and so they removed the $1200 that has been added to your return to make up for a missed payment. If you had already received your first stimulus payment, then your answer on TurboTax should have been that you were not missing a payment and then Line 30 would have reset to zero.
This indicates that you told TurboTax that you received the $600 payment, but not the $1200 payment. The IRS checked and is indicating that you did receive the first stimulus payment.
