This has more to do with the Speedometers of HGVs being more accurate than those of a car. HGVs have tachos so are accurate, Cars can legally be -10% (average about 5%) so your speedo says you are doing 50 but are probably doing 46-47. The HGVs being time limited, they tend to want to drive at the max speed they legally can, yes I know in reality this is seconds but that's human nature.
Average SPeed check zones usually mean inside lane for me, I couldn't give a shite if I'm driving at 45 in a 50, I prefer to live to tell the tale