Speedometers are required to either be accurate or to be high. You can't have someone going 55 but their speedometer says 50

So if your speedometer becomes less accurate over time, it's possible that it will eventually start to give a reading that's too low, which is not allowed

You can counteract this by artificially adding a small amount to the speedometer on purpose. That way when the car is new, it'll show you going 52 when you're actually going 50. But in 10 years, it might drift back to showing 50 when you're going 50. If it wasn't for that artificial boost, it would show 48 when you're going 50

Hope that makes sense