Weird behaviour for sent emails

[Jonathan5]

This is a pure guess, but maybe a user has muted a category, topic or tag (and maybe there is no log of that reason for not emailing).

I had a similar experience but never looked into it in detail in the end:

Data explorer query - users' email settings

How could I find out how many users have each email setting? I’m asking as only about a tenth of users watching a topic are being emailed post notifications. Thank you.