I received a letter from the IRS notifying me of a Tax Withheld difference between my 2019 Form 1040 and information from other sources (i.e. my employer). Process?

When I did my 2019 taxes, I uploaded my employer W2 automatically in TurboTax.  Line 17 has 33,808.  The letter indicates that 30,534 was reported.  How could a difference have happened?  The IRS guided me to submit a 1040X.  Is this straight forward to resolve?