The number of FB comments displayed is not the same as comments in the FB Comments plugin.
RebelMouse uses total_count value (from Facebook comments) to display number of comments, but the Facebook Comments plugin that displays the comments count can use a different number due to privacy settings.
This is how FB counts comments and RebelMouse displays the FB value. This includes some comments which are no longer visible, but were accessible in the past. These no longer visible comments may be edited (privacy settings) or removed. This is standard practice for media sites.