Computer isn't assigned to site as expected

Apologies for the beginner’s question. I have a site for non-Windows computers.
I have come across the system where the site subscription relevance evaluates to “True”, but the system is not subscribed to the site.

Thank you,
Aleksandr

==== from the site subscription tab ====
Subscription Group
This group includes computers with the following property:
Relevance expression is true
not exists (operating system) whose (it as string as lowercase contains “Win” as lowercase)

==== from the qna ====
Q: not exists (operating system) whose (it as string as lowercase contains “Win” as lowercase)
A: True
T: 4445

Q: operating system
A: Linux Red Hat Enterprise Server 7.5 (3.10.0-862.14.4.el7.x86_64)
T: 66