Apple Watch tracking sleep even when not in use

Hi all,

I left my watch ultra 2 charging all night and for my surprise, next day, the app tracked my sleep. With all 4 phases is detail. How can this be possible since I wasn’t wearing it? I lost my trust on these watches. Why is the watch tracking my sleep if it is not on my wrist?

[Re-Titled by Moderator]

Original Title: sleep tracking while not using the watch