The tax caster tool starts out by putting the 3rd stimulus amount into the estimated “refund” you will get because not everyone received the 3rd stimulus in 2021—-some people will not get it until they file 2021 returns. So when you enter that you actually already received the 3rd stimulus, the estimated “refund” is reduced because you cannot get that stimulus money twice. You are not being taxed on the stimulus and you are not being cheated out of it.
In January you are going to receive a letter from the IRS that shows how much stimulus you received in 2021. You will enter that information carefully into your 2021 tax software. If you are eligible to receive any additional stimulus money then it will appear on line 30 of your Form 1040.
So.....it sounds like you entered into the actual tax software that you already received the $1400 3rd EIP. You cannot get that payment twice. That explains the difference between the estimate you got with the estimating tool and your tax return.
Correct...you failed to enter the advance stimulus payment in the taxcaster tool correctly which explains the difference.