It could happen because you used the actual expenses method in prior years. If you want to use the standard mileage rate to calculate vehicle expenses, you must choose it in the first year you use the car for business. Once you use actual expenses for the vehicle (even if it's the first year you used it for business), you can't switch to standard mileage rate.
You can find more information about vehicle expenses here:
Standard Mileage vs. Actual Expenses
