Did they actually click into the blog to read the post or did they read it in their Reader? There are several different ways that readers can start following your blog.
A follower can click the “like” button when viewing your posts in the Readomatic reader on the home page of WordPress.com without visiting your blog or on The Topics page and that means no visitor stat is created. An email subscriber can click the “like” button in the email they receive of your new posts without visiting your blog.
P.S. To compel readers to click into my blog to read the full post I have set my RSS Feed to “summary” on this page > Settings > Reading