Did not received full amount of 3rd stimulus. Recovery Rebate credit 2021-

I do married filling jointly. I received a letter 6475 stating  total 2021 EIP = 2100$.
(my understanding is : we should get 2800 together - 1400$ each as per eligibility requirement for 3rd stimulus)
while filing with turbotax- it auto-populated as a diff of 700$ on line 30.
i filed the tax- received a notice CP-12
it does not give clear explanation on recovery rebate credit and only showed exact same calculation as filed.
Any advise please.
thanks!