TÜV badge issued retroactively – right?

If you bring your vehicle to the TÜV only later than after two years, e.g. after 2.5 years the badge is “re-dated”. Actually I bring a vehicle to the TÜV so that the traffic safety is determined. The badge is granted if the 2 year rhythm is observed for another 2 years. However, if I cover the 2 years by 1/2 year, I get the new badge only for 1.5 years. Thus it has now happened to an acquaintance. The facts of the determination of the traffic safety However, it does not change and therefore the plaque should not be awarded for a shorter period of time. In my opinion, the plaque should also be issued for 2 years. Does anyone know on which legal basis this is based? Are there possibly any court rulings to which one can refer?