Or more to the point, how they think they can get away with it, or clearly do all the time?
I do a lot of motorway driving and I see so many cars doing over 80!
See also dual carriageways near me. People do 80+!
I'll be in a lane sometimes doing 70 and I have cars behind flashing at me to speed up 
My question is do people not get done very much for speeding? I wouldn't really think to speed anyway but surely the strong possibility of a fine or more is a deterrent? Do people just not care?