The concept of a user is hard to define. FastStats Analyzer uses the widely accepted I/PRO method of calculating the number of users who have visited. Each unique IP address visiting your web site is considered a user, and a request from that same IP address over 30 minutes after the last request also adds to the user count.
If your site uses a session cookie, you can set up Analyzer to read this cookie value, in which case Analyzer will incorporate the value into the session identifier along with the IP address. You will get a much more accurate breakdown of sessions this way.
The best solution is to use a visitor ID cookie, so that your site can assign an unique identifier to each visitor.