What causes speedometer not to read right speed traveling?

Speedometers are not allowed by law to under-read, but are allowed to over-read by no more than 10%. So at a true 30mph, they can legally read anything between 30mph and 33mph.

Because they read the speed via a sensor in the transmission, not the true speed of the vehicle across the ground, they have to be calibrated such that they under-read slightly when the vehicle is lightly laden and has brand new tyres fully inflated. As the tyres wear, and if they are slightly under-inflated, or compressed by a greater load in the vehicle, the speedometer will over-read by more.