The wind direction angle changes 15 degrees, why can't the wind vane run accurately?

Wind direction judgment: the wind blowing from any direction is called wind. For example, the southeast wind is the wind blowing from the southeast. The weather is often F, which is the north wind level 4; 1 is the wind pole, the second is the wind vane, the first level is level 2, the second level is level 4, the third level is level 6, and the eighth level is represented by a small pennant. What about the wind direction? Please compare-windmill:+to judge!