Each user on the Internet is generally assigned a unique 32-bit number, called an IP address. IP addresses look like this: 206.102.200.1. Internet Service Providers (ISPs) who provide dial-up connections generally dynamically assign IP addresses to their users, so individuals coming to your site will not each have a unique IP address.
This is why FastStats Analyzer uses the I/PRO algorithm when calculating the number of visiting users.
To improve the ability of Analyzer to recognize separate users and track separate sessions, you can make sure your site sets an appropriate Visitor ID cookie, or at least that it sets a session cookie for a Session ID.