Frequency has abstract mathematical meaning. If the occurrence of an event cannot be predicted in advance, then the probability of occurrence can only be estimated by means of mathematical statistics through a large number of measured data. This probability is called empirical frequency, which is called "frequency" in hydrological calculation.
For the sake of popularity, "recurrence period" is often used instead of "frequency", which refers to the average time interval during which an event repeats in many experiments. What needs to be pointed out in particular is that the so-called "return period" does not mean that it happens just once in many years, but has the significance of statistical average. More precisely, it refers to the number of time intervals required for the average value of a hydrological variable to be greater than or equal to the specified value.
The above is taken from Baidu Encyclopedia.
In other words, this is just a probability event, just like drawing lots. If you smoke one the first time and then want to smoke this one, you may smoke it right away, or you may have to smoke it many times to win. This is all possible. This is not abnormal, it is just a question of probability.
However, it is worth mentioning that human activities have had a great impact on the earth's environment, such as global warming, soil erosion, desertification and so on. This has led to the change of the earth's climate and increased the frequency and probability of extreme weather. It is just a problem that all mankind must pay attention to.