After a little search, I found that the website said that adding a bar to the letters meant that the average value should be derived from Maxwell.

In Maxwell's book "On the Dynamics of Gases" published in 1867, Maxwell put a bar on the letter V representing speed to indicate the average speed.

For a long time after that, adding a bar to represent the average value was very popular among physicists.

But the British mathematician karl pearson is a statistician with a strong physical background (the founder of mathematical statistics). He applied physical habits to statistics, and since then this habit has become quite popular in statistics.

Thanks for adoption