It says UNIX because you're changing the password for authentication provided by the
pam_unix PAM module. Quoting the manpage:
pam_unix - Module for traditional password authentication
This is the standard Unix authentication module. It uses standard calls
from the system's libraries to retrieve and set account information as
well as authentication. Usually this is obtained from the /etc/passwd
and the /etc/shadow file as well if shadow is enabled.
It is related to Unix in that it uses the traditional files
/etc/shadow as the source of authentication.
If you used another PAM module, like
pam_ldap for LDAP authentication, it would say LDAP instead of UNIX:
(current) LDAP Password:
Retype new password:
passwd: password updated successfully