The information you get from both Zillow and Trulia are not provided in real time. At some point the information may have been correct, but the are stale as time pass by.
You can get accurate property tax information from the State website if they are on line. In the State of Maryland as well as the District of Columbia tax records are readily available either through the specific County or the State.
Regardless of jurisdiction, a Real Estate agent should be able to provide you this information