Determining the number of “unique visitors” is truly a bit of web “black magic - voodoo - prestidigitation”.
There are a lot of things to consider when tryhing to determine if a give page request is from a “unique” visitor - hence programmer employ things like sessions and cooklies in an attempt to sort all this. Your traditional "stats’ package can, at best, only guess at whether a page request is “unique” without such programming in place (usually by making some evaluations based on IP address and time).
A “successful request” is very different, and only indicates that a request (whether for a page, or a page element) was successfully completed. In the DH provided stats, there is an accounting of successful request for pages, as opposed to just a successful request (for an image in a page, for example), but this does not indicate a uniqueness of the visitor/computer making the request in any way.
There is a lot of information about the DH Stats in the wiki, and you can google for additional information on how to determine unique visitors. Good Luck!