The coil inside expands or contracts in response to the temperature. This turns and makes the needle move.
If I'm not wrong, the hygrometer works by comparing the temperature change due to the evaporation into the air. That is to say, when the air humidity changes, some evaporation occurs to keep an equilibrium- this results in a slight drop in temperature.
At least that's what I think after staring at it. Alternatively, it could also just be some silly mix-up at the factory. I can't think of any other way it could work at the moment.