This means you tried to enter in the new password when it was asking for CURRENT again for confirmation.
When you use the 'su -' command, you are prompted for the root password to log into the root account. The first time you do this, you are forced to immediately change the password. To authenticate, it immediately asks you to re-enter the CURRENT root password. This is still the 123456 default. After that you can enter your new password (twice) and the root password will be set to the new value.