When you choose to create a Custom Audience based on your Facebook fan page, you have several analytics choices available to you:
People who currently like or follow your page
Everyone who is engaged with your page (what we usually recommend)
Anyone who visited your page
People who engaged with any post or ad
People who clicked any call-to-action button
People who saved your page or any post
Depending on what you choose as an event, you will get a different audience size. Therefore, don't always rely on the number of likes on your page, check what you have chosen as a starting setup by creating your Custom Audience based on your Facebook fan page.