Apparently there are two reasons for that:

1. When the manufacturer carries out speed calibration tests, they're based on brand new tyres. “But over time, as the tyres experience normal wear and tear, they get smaller in circumference. This changes the accuracy of the reading of the speedometer – again showing a higher speed than the actual speed.

2. Speedometers are calibrated at the factory to read at or above the actual speed of the car. The goal is to avoid sending out cars that have speedometers that indicate you are going the speed limit when you are actually exceeding it.
The idea is to err on the side of caution. The same thing is done with gas gauges. The gauge is always calibrated to show empty when you actually have a gallon or so of gas left in the tank.
These practices are designed to give the driver a little cushion. Doing so keeps people out of trouble. Human nature is to push the limits, whether your speed or how far you can drive before filling the tank.