Only the IRS would know what and why they changed your return/refund. You should receive a letter from the IRS explaining what they changed and includes information if you don't agree. The letter can take 4 to 8 weeks to arrive.
Once a tax return has been Accepted by the IRS or a State, TurboTax receives no further information concerning the tax return or the status of any tax refund.
What is the difference reported on the 2021 tax return and the amount the IRS shows as being refunded?
One of the reasons for the IRS to reduce the refund is due to the Recovery Rebate Credit.
If your refund is short then that is because you said you did not receive the 3rd stimulus payment and the IRS has records that show you were sent the 3rd stimulus payment so they removed the Recovery Rebate Credit that was entered on Form 1040 Line 30.
Go to this IRS website for your tax account to see if the 3rd stimulus payment is entered - https://www.irs.gov/payments/your-online-account
If you are Married Filing Jointly also check your spouse's tax account.
If you checked your bank account in the March - May 2021 timeframe and there is not a EIP3 payment from the IRS then go to this IRS website for how to start a trace on the payment - https://www.irs.gov/newsroom/questions-and-answers-about-the-third-economic-impact-payment-topic-j-payment-issued-but-lost-stolen-destroyed-or-not-received