I think, the fact that it's documented that doing 71 in a 70 *can* (or *will* - as written down in law) earn you a fine & points is the problem for me. Whether or not allowances are made, or cases have or have never been seen is by-the-by, the simple fact is that someone, somewhere could enforce that should they so desire.
I get the whole calibration thing, wheel sizes, speedo's (not the budgie smugglers), moving shadows for time & distance calc .. all that stuff, but all that aside, if some guy or guy-ess decided that you doing 71mph was enough to earn you points (either the copper or higher up the chain), then it could be done. I'm not saying that it ever has, but it *could* be done.
My daughter is hoping to join the police force (as a traffic cop!) so I've go no axe to grind about they boys and girls in blue, after all, they're only there to enforce the laws that someone else has dreamt up.