Could be miscalibrated radar settings, that's why there are markings on the road though. If the photos taken don't match what the radar system says you shouldn't get a ticket, and if you do then then markings will clear you.
The calculation is distance divided by time = speed, the distance is determined by the gap between the road markings and the time is the time between flashes, usually 0.25s for a GATSO camera. For example if you travel 100m in 1 second then your speed is (not surprisingly) 100m/s (Which would guarantee you a speeding ban being 223mph) a calculation is available
here