Wart, I'm with you, I think it's a legal thing. I'm pretty sure those fancy speed guns have a tolerance as well which is why you're fairly safe within say 9 mph over the limit. With both tolerances you could argue almost any ticket you got for 1-9 miles over the limit and win.
My wife fought a ticket earlier this year (as her own counsel and did very well). She brought into question the accuracy and tolerance of the radar gun. She actually had two tickets, one for illegal lane useage (driving in left lane) and speeding. The judge must have been impressed because he did drop the illegal lane change and was smiling the whole time, like he wanted to drop both but couldn't bring himself to it.
"By failing to prepare, you are preparing to fail." -- Benjamin Franklin