Why, today's weather forecast says it won't rain, but it does?

The accuracy of weather forecast seems to be only 80%~90%. In addition, some air masses may form rapidly overnight. Some air masses may suddenly meet another air mass, such as a warm air mass and a cold air mass, and suddenly it rains. What's more, sometimes, like the staff of the weather forecast bureau, some air masses or clouds are blocked by mountains and can't reach your place. But some of them passed. It's hard to say that it will rain. . .