We've filed through Turbo Tax in 2018. For our 2018 State Refund reported in 2019's return, Turbo Tax auto-populated an amount different from the actual 2018 refund. Why?

It is not uncommon for the amount that the program calculates as a state tax deduction differs from the actual return.  This can be due to something omitted from the tax return, or a data entry error for example.  In such cases, TurboTax will auto-populate the amount the program calculated rather than the actual amount received.