When would they do this?

Often times the amount of tax a U.S. taxpayer actually pays to a foreign country is not the same as the amount he accrued. IRC § 905(c) requires that an adjustment be made to the foreign tax credit when the amount accrued differs from the amount paid. The IRS will re-determine the U.S. tax when (a) the accrued taxes differ from taxes paid, (b) the accrued taxes are not paid within two years after the tax year closes, or (c) any tax paid is refunded in whole or in part. In addition, the Treasury Regulations provide for a number of redeterminations.