Its speedo error. Most cars and motorbikes slightly overestimate speed, and the effect is more pronounced at higher speeds. This is just to be on the safe side with regards to speed, as tire tread, tire size, temperature, etc. can all have positive or negative effects on speed. Its usually around 5%. You can test this by driving at a constant speed near those radar speed detectors and observing the speed on the board vs. your speedometer. My speedometer is usually about 4 mph faster at 50 mph.
Note that those road side systems are usually only used as a road sign and the vast majority (at least in California) use a cheaper and less accurate/repeatable sensor than the ones used by police for speed infractions. This is especially pronounced on highways where the same sensor can give you several different readings at 1-10 mph less than actual speed.
