Pitot tubes are notoriously innacurate in anything other than forward flight, and they also vary from aircraft to aircraft. As an example - when demonstrating a low or zero speed autorotation in a jetranger, you can reduce airspeed to zero indicated - the aircraft will now be descending almost vertically (if there is any headwind the track over the ground will actually be backwards). As therate of descent increases, the pitot tube experiences disturbed airflow as a result, and the indicated airspeed will then actually start to increase throught 40kts or so. In actual fact the real airspeed is still zero or possibly even a negative value ie backwards !!
Similarly it is not impossible to be sat on the ground (or in the hover) with a 30+ kt crosswind / tailwind and the indicated airspeed will be zero.
I would guess that the only time you get anything like an accurate reading is in forward flight, straight and level and in undisturbed (ie non turbulent) air.
Cheers
Jay