There is a glitch in the Family Tax Cut paragraph of the standard Client letter. It is picking up the amount from line 22 of S1A instead of line 23.
Easy fix if you have customized your letter:
Change the calculation to report the reduction in Federal Tax payable from:
{{format(S1A.FamilyTaxCut.M[3])}}
to
{{format(S1A.FamilyTaxCut.M[4])}}