Looks like it's a deliberate thing to try to keep drivers at reasonable speeds.
no its deliberate to stop the manufactures being sued by people who got ticketed for going over the speed limit and blaming the manufacture for having a speedo that under reads.
they read faster on purpose so you cant be going faster than its actually displaying.
Also, really what material difference does 5kmh or so make to anyone?
What is anoying is that the car knows how fast it is going quite accurately. The stability control needs the info to be spot on. In My BMW I was able to code out the diliberate speedo over-read and it is now bang on