The innacuracies are in the new guns and their “pop” mode. Basically it is an ultrafast start and shutdown mode for the gun. The reason is, of course, RADAR detectors. They’ve gotten quite good. They don’t necessarily need the gun to be on and transmitting to pick it up. When the gun is in standby (with it’s electronics operating but not transmitting a beam) they can still be picked up. Same sort of way RADAR counterdetectors work. Even though the detector itself isn’t trying to emit anything, it does anyhow (as does any superheterodyne device).
Ok, great, however you might pause to wonder about the ability to electronics operating in the 30GHz range to quickly come on and stabilise and, well, you’d be right. Guns in “pop” mode aren’t accurate. In part due to the fast start, in part due to less data points, they can produce unreliable readings. The gun manufacturers say that pop mode isn’t to be used as a final speed measurement, but that doesn’t stop police forces from doing so anyhow.
Or it could be even more simple: The gun wasn’t calibrated. Like any precision device, they need periodic recalibration. Had this been allowed to happen, it is entirely possible the gun was producing inaccurate readings.
It is a good idea for all drivers to take a little time to educate themselves about various speed measurement technologies and such. While I’d say the majority of police departments use their equipment right and the tickets are legit, they aren’t always. If you get nailed with a bogus ticket, you don’t necessarily need GPS to fight it. Tell the department you want the calibration records for the gun in question, find out if it was in pop mode, etc, etc. If they screwed up, let the judge know and they’ll most likely drop the ticket.