Per the Iowa 1040X Instructions
we must use the tax rate schedules to calculate the tax and cannot use the tax tables. Due to this there may be a small difference between the tax calculated on Iowa Form 1040 and Iowa Form 1040X.
Page 3 of the instructions states:
: Enter the correct amounts in the appropriate line items. These entries can be referenced to certain lines on the original return by use of the Line Number Reference Table and the line instructions below. Tax rate schedules are provided on page 2 of these instructions and must be used in calculating the tax due on line 10 of the IA 1040X. Do not use the tax tables in the IA 1040 or IA 1040A instructions.