Why is the total due on my Preliminary tax bill different from the total due on my Actual tax bill?

The Preliminary tax bill is an estimated bill based on half of your net tax of the preceding tax year, or, if your value has changed, half of the calculation of your new value multiplied by the tax rate of the preceding tax year. The Actual tax bill is the balance due as a result of the tax calculated by multiplying the new value by the new tax rate, minus all payments received on the Preliminary tax and minus any exemptions.