Heh. Since its not a fixed camera, they aren't (usually, though it is possible depending on placement), directly in line with your direction of travel. Thanks to trigonometry, any reading taken from an agle away from the diection of travel will be less. greater the angle the less it is. Eg, if your coming up to a van, and your 400 meters away, and its right on the side of the road, it'll clock you doing pretty much what you are doing. However, by the time you are right next to the van, your speed towards the van is pretty much zero.
Not sure if the cops are allowed to compensate for that error (however, i'd assume that fixed speed camera's have that caculated in automatically)