now that we know that you are looking for "trick" answers:
atmospheric pressure, tyre pressure, temperature, manufacturing deviations in tollerances, etc can also play a role. do note that 99.9999% of cars have innacurate speedos from the factory. then again these are locally made cars,.. anything is possible!
if you really want to know how fast you are going, ask a motorcyclist, their speedos are MUCH more accurate than car ones.