Any tax withheld was only an estimate. Your actual income tax owed is calculated from your total income, deductions and credits, and you get credit for any tax payments and withholding.
You reported 1/3 of the distribution as taxable income, but you got credit for all the withholding. Partly, that's because the law allowed the distribution to be spread out over 3 years but did not contain a provision for the withholding to be spread out over 3 years. And you wouldn't want that anyway, since if the IRS kept your withholding for 3 years before applying it to your tax account, you would be giving them a large interest-free loan for that time.
As a result, you paid tax on 1/3 the distribution but got credit for all the withholding. Your 2020 tax bill was smaller, or your refund larger, than it otherwise would have been. In 2021 and 2022 you will again calculate the income tax you owe based on 1/3 the distribution plus the total of your other income, deductions and credits. You don't get credit for the withholding because it was "used up" on your 2020 return.
