Speed detection displays

[jj4091]

Noticed this a few times and always been puzzled about how these things measure your speed. If I'm driving behind a high vehicle ( wagon or even van) as the vehicle in front of me passes the display and then I pass in my little Suzuki the reading always increases by 2 or 3 mph even though we are both driving at the same speed. It doesn't seem to happen in a line of cars as far as I can tell.
It's usually a 30 mph sign. Could this be correct or is it my imagination.
Just happened again on my way home which has prompted the post

 

[Wayners]

I'm going to guess. I don't actually know for fact. Maybe you can look up online and give an opinion.
I think the further you are away then the better the line of sight for the radar.
As the lorry passed it could be showing slower than the actual speed, then when you be become visible the speed display jumps up because the radar is bouncing better and more accurate
There is a optimal distance for accelerate radar reading and it's not close or by the side of the radar from what I understand