WordPress.com stats are based on page views. The metrics provided by that program do not provide us with stats for unique visitors. A page view is defined below.
If for example sake, I click into your home page in the morning and do not click any links on that front page then counts is 1 page view. If I click in to your front page later in the day and click links to 20 posts on your blog then the page view count is 20. See > http://en.support.wordpress.com/stats/
What is a hit?
In web analytics, a hit is any request for a file from a web server. By request means a hit calculates page content delivered, all images to complete that page, and any additional files that need to be loaded to make the web page you are looking at, appear the way it does.
What is a page view?
A page view is a request to load a single page of an internet site that results from a page request from a web surfer clicking on a link on another HTML page which is pointing to the page in question.
What is a unique visitor?
A unique visitor is access from a single IP to a web server that generates page views and hits during a particular visit. When a visitor has cookies disabled, there is no way of establishing if they are a unique visitor or not.