Well, as I recall from the ending days of my Criminology class a few years ago (we were ending the semester and had more or less run out of things to talk about), the reason for that is the calibration of the radar the cops use.
As I recall, the State Cops, at least in Pennsylvania, have their radar guns calibrated to within about 6 MPH, so that's the minimum over-the-limit speed they can get you for. The local cops' radar varies from 7 to 10 MPH.
Also, as a fun fact (again, at least in PA), the radar gun has to have been recalibrated within the last 30 days, or else it's considered unusable for ticketing purposes.
All this info is correct as I remember it; however, if I posted something wrong, tell me.