Its not that uncommon that some people don't know how to use every day stuff correct and use them wrong. One thing I've noticed that a lot of people use wrong are thermostatic radiator valves. Its not a simple valve and the numbers don't indicate on how far open the valve is, its a temperature setting. I've often told people to not set it to 5 and rather set the target temperature and 3 is about 20°C, so room temperature. At work all engineers don't get it and we stupid little IT guys with a smaller degree get it right.
To be fair, they aren't that accurate. Its pure mechanical and the sensing happens in the thermostat but when it closes the valve, the radiator is still full of hot water. You need to find the correct setting for your room and so on. With electric ones, you can fiddle with the settings, to be more accurate.
Also, they measure right next to the heat source and the markings can't compensate for better or worse insulation. It makes (almost) no sense to put temperature markings on them.