Personally I agree with you about EVs getting a break to incentivize more buyers to purchase and due to the benefits of EVs (reduced emissions, improved safety, less wear and tear on the roads from fuel tankers which are giant rolling HAZMAT bombs etc etc). However, the law is the law. If you don't like it, lobby your local State representative to change the State code or your local Board of Supervisors to enact EV credits towards personal property tax. I know we should pay our fair share to support road infrastructure etc but the annual tax (based on assed value) on a vehicle with such expensive tech is illogical and ignores the societal costs that ICE vehicles cause.
That said, there was a letter from the Fairfax County Director of Tax Administration from 2014 posted upthread
#47
which basically outlined how they set the value of a vehicle for taxation purposes. They use a "recognized pricing guide" (such as N.A.D.A.) to look at the value base model of the vehicle,
without options. If that is not available (not published in the guide), they apply 95% to the MSRP the first year and 90% the second. I would suggest you look at the assessed value of the car on your tax assessment and see which of these rules were used to determine your assessed value. I have personally experienced some assessment errors with both my Model S and Model 3 which I appealed to the locality (Loudoun County for me).