Groups membership not sticking Centos 7

I've ran into something so strange I'm at a loss of even how to explain it so I'm going to demonstrate it
[siggib@localhost ~]$ whoami
siggib
[siggib@localhost ~]$ groups
siggib
[siggib@localhost ~]$ su -
Password:
Last login: Thu Nov 8 08:25:35 PST 2018 on pts/0
[root@localhost ~]# groups siggib
siggib : siggib wheel
[root@localhost ~]# su - siggib
Last login: Thu Nov 8 08:13:53 PST 2018 on pts/0
[siggib@localhost ~]$ whoami
siggib
[siggib@localhost ~]$ groups
siggib wheel
[siggib@localhost ~]$ exit
logout
[root@localhost ~]# exit
logout
[siggib@localhost ~]$ whoami
siggib
[siggib@localhost ~]$ groups
siggib
[siggib@localhost ~]$

Why is the user in the wheel group when viewed from the root account or when root su to it but not when it is just plain?

It sounds like you didn't log out and log back in after being added to the "wheel" group. Your session acquires its credentials when it starts. Subsequent changes to /etc/groups won't affect it.

duhhh Thanks, how did I miss that :-(

Moved: This thread is more suitable in <CentOS> and has been moved accordingly to help your thread/question get the exposure it deserves.