FAQ: Visitors vs. Session Users
Why doesn't the number of visitors in the Overview match the number of session users in the "Sessions Overview" report?
Session information only shows users contributing page views, and other views show all visitors. Also, long sessions are discarded from the session information.
The configuration database is split into two major sections: the main statistics, and the session information. The main statistics contains information on all hits; the session information shows the "sessions" -- i.e. it tracks the sequence of page views of each person who visits the site. Most views show the main statistics; only the session-related views (Sessions (summary), Sessions, Paths (clickstreams), Paths through a page, Entry pages, Exit pages, Paths through a page, Session pages, etc.) show the session information. Because these two types of data are computed differently, the numbers may vary between the two.
There are two major factors that affect the session users, but do not affect the visitors. First, session information is based on page views only, while visitor information is computed based on all hits in the database. So for instance, if the web site is accessed by a browser that fetches only a single image file, and never hits a page, that hit (and that host) will appear in the main statistics, but not in the session statistics. To put it another way, the visitors are the number of unique hosts who contributed hits; the session users are the number of unique hosts contributing page views. If your database is set up to track hits or bandwidth, these numbers may be significantly different. if your database tracks only page views, then visitor information will also be based on page views, and visitors and session users will be closer.