I am able to use following combinations of user/password while using installer:
 - foo/foo
 - user/a

however add-user script is more restrictive:
 - foo/foo:
  * Error * 
JBAS015238: Username must not match the password.

 - user/a
 * Error * 
JBAS015269: Password must have at least '8' characters!

 - user/aaaaaaaa
 * Error * 
JBAS015266: Password must have at least one digit.

 - user/aaaaaaa1
 * Error * 
JBAS015267: Password must have at least one non-alphanumeric symbol.

There should be some consistency between adding user via installer and adding user via add-user script.
There is already task created for this issue see BZ900725