User not in sudoers, but still can use sudo

I have a user user1 that is not in the sudoers file (nor belonging to groups in sudoer) but can still use sudo. How is it possible ?

user1~$ groups
        user1 www-data

My sudoer conf

        root ALL=(ALL) ALL
        %sudo ALL=(ALL) ALL

        #includedir /etc/sudoers.d

user1~$ sudo -l
        User user1 may run the following commands on this host:
        (ALL) ALL

As you can see, your sudoers file contains a line above which includes a number of other files from /etc/sudoers.d. You may be thinking that the hash symbol in front is a comment, but from man sudoers:

It is possible to include other sudoers files from within the sudoers file currently being parsed using the #include and #includedir directives.

You will need to show us the contents of all files in /etc/sudoers.d/ before we can comment on why user1 has sudo privileges.