It is believed that about 6,000 stars are visible in the entire sky with the naked eye, of which about 3,000 stars are in the Northern Hemisphere

It is believed that about 6,000 stars are visible in the entire sky with the naked eye, of which about 3,000 stars are in the Northern Hemisphere of the sky. In our time, many millions of stars, including very faint ones, have been carefully recounted and cataloged. Why is the number of stars that can be seen with the naked eye in the Northern Hemisphere of the celestial sphere never indicated exactly?

There are several reasons: 1) visual acuity is different for different people; 2) the conditions in which the observations are carried out are different each time; 3) there are variable stars that change their brightness, becoming periodically inaccessible to observations by the eye.