I'm trying to understand how Facebook advertising calculates the reach. Here is the issue: I have 10.000 fans from my country, but when I select "show only to my fans", the reach is estimated do 6.000. Why so?
I would guess not all of your friends allow friend's of friends to see their comments on your posts, or have their profiles exposed to google/everyone, this seems to be the basis of the reach data.
It's also possible that people who have 'liked' your page have unsubscribed rather than unliked your page, this would also affect your reach.