I found, that when I check some users, thay havn't assigned any group, via API they belongs to autoconfirmed group.
Example:
https://pl.wikipedia.org/w/api.php?action=query&list=users&usprop=groups&ususers=Ziomal1234
https://pl.wikipedia.org/w/index.php?title=Specjalna%3AUprawnienia&user=Ziomal1234