How much is missing? The 2 most common reasons this year is you entered the wrong amount on line 30 for the Recovery Rebate Credit (3rd Stimulus payment) or line 28 for the Child Tax Credit. Those lines are just if you didn't already get the full amount or qualify for more. The IRS knows what they already sent you.
TurboTax calculates the amount of your refund, or additional taxes owed based on the information that you entered into the program. When the IRS and/or your state receives the returns, they compare what is on the return to what they have in their system for you.
When they make adjustments to your return it is to account for the difference in what was reported on the return and what they are showing in their system.
You can call the IRS and speak to a tax representative about what was changed on the return and why.
You can also see this Turbo Tax FAQ for information.
Why is my direct-deposited refund or check lower than the amount in TurboTax? https://ttlc.intuit.com/community/refunds/help/why-is-my-direct-deposited-refund-or-check-lower-than-the-amount-in-turbotax/00/25594