The program doesn't show number of "unique visitors". It determines number of visitors by the IP addresses. If a request from an IP address came after some time (timeout) since the last request from this IP, it is considered to belong to a different visitor. The timeout is set to 30 minutes by default but you can change this value in Options > Analysis > General. Requests from spiders aren't used to determine visitors unless the "Count spiders as visitors" option is enabled.
So referring sites are shown for usual visitors, not unique visitors. In most cases this referrer is determined referrer of the first page viewed by a visitor.
So referring sites are shown for usual visitors, not unique visitors. In most cases this referrer is determined referrer of the first page viewed by a visitor.