YouTube video views decreasing. Why there is a view dropping ?

I have found that sometimes there is a view decreasing in case of my videos uploaded on YouTube.

One video reached more than 100 views, but a couple of days later, the total number of views is less than 100.

Why does it happen ?

The number of video views is counted instantly. But later, there is an audit of views, that checks if the recent views are legitimate. If there are some views that appear to be added by an unusual activity, they will be deducted.

The number of YouTube video views can be dropped:
- If someone watches the video only for a few seconds (if the video is closed shortly after it is opened)
- If video was opened by some bot (for example because of the crawling)