Why root user is unable to change a password of normal user's ?

Issue

• Why root user is unable to change a password of normal user's ?

• While changing password of normal user as root user, passwd command gives  following error.

  [root@example ~]# su - mysql
  -bash-4.1$ passwd
  Changing password for user mysql.
  Changing password for mysql.
  (current) UNIX password:
  passwd: Authentication token manipulation error