I think a lot of vehicles are like this. Think of all the vehicles that are going 52 down a two-lane highway. I drive 30 miles per day on a 2-lane highway, and it's amazing how many people are going just a little bit under the speed limit. My '96 Beretta was about 4% over. Bigger tires fixed that
I wonder if perhaps it was an attempt to stop casual speeders? It wouldn't be that hard to make a spedometer register correctly, ESPECIALLY if the odometer reads correctly.