The post count is calculated differently on the About page than it is on the Admin Dashboard. On the About page the post count is calculated with Post.count. The query that is used for the 7 day and 30 day periods is similar to this. It will return all posts, including PMs. On the Admin Dashboard the post count is calculated with this:
Post.public_posts.where(post_type: Post.types[:regular])
This will exclude private messages from the post count. It will also exclude moderator action, small action, and whisper posts from the post count.