thank you..
that magic number of 21 has been overused..
The number should be understood as “up to 21”.
Not as a guarantee that a patient will get that long, just meaning that long must pass to be sure the disease is not still undetected in a patient who was exposed.
No it hasn't.
Every human being is different. 21 days is a very conservative estimate of a 95% confidence interval that you will develop symptoms if exposed, within two standard deviations. The measured standard deviation is 4.3 days. For "most" people (~70%) symptoms will develop between 8 and 12 days of infection.
If you stop isolating people after only 12 days, 30% of infected patients are going to be turned loose into general population. Some studies are now recommending 25 days.