Thanks for the quick reply 
Seems like user:list ist not a valid command
sudo -u www-data php occ user:list
[Symfony\Component\Console\Exception\CommandNotFoundException]
Command "user:list" is not defined.
Did you mean one of these?
config:list
files_external:list
app:list
encryption:list-modules
user:resetpassword
user:delete
user:lastseen
user:report
user:add
And user:report does not show the info I’m looking for - and confuses me even a bit more
+------------------+----+
| User Report | |
+------------------+----+
| Database | 8 |
| | |
| total users | 8 |
| | |
| user directories | 17 |
+------------------+----+
Without duplicates, I currently have 12 accounts where 2 are not via IMAP authentication - but those numbers do not match at all 