This is due to the software’s resolving of a bit of circular logic.
Federal taxable income depends on the state tax, because state tax is an itemized deduction.
But in some states, state tax depends on federal taxable income.
So the computation is circular.
Here’s how the software resolves this: For the calculation of the state tax for itemized deductions, federal tax is computed without state taxes. This makes the two tax numbers different.
Note that in the real world, the numbers are always different, because the itemized deduction is based on the cash flow of taxes paid, not the state tax finally due in a given year.