You can't apply this methodology to accounts belonging to such large celebrities for one important reason: when registering for a Twitter account you're prompted to follow "recommended" people. Someone could be told about Twitter, register an account, click "follow all" on the recommended page and then never sign in again... Justin Bieber would have gained a legitimate follower but they wouldn't seem real.

This methodology can only really be applied to smaller accounts, ones that have not had exposure through the Twitter recommended users system.

Here is an example, after creating an account you go through an introductory process that requires you to follow at least 5 people to continue: http://i.imgur.com/xT0tc5c.png and then on the next page you have to follow another 5.