Answer: The visits tracks every unique session to the server. A session does not require a particular page to be requested. Consider a site is listed on Trulia, every time the listing comes up on the Trulia search a picture will be displayed on the Truila search result. The picture request coming from Trulia does not request a specific page, but it is a unique visitor requesting one of the property photos. It would count as a visit at the site level but would not count as a visit for a particular page.
Key Terms:
Visits: A count of each time someone viewed your PowerSite. They can look at dozens of pages, download documents, floor plans etc. but this still counts as 1 visit. That same person could view your PowerSite later the same day and would count as another visit.
Page View: Is a count of each PowerSite page displayed. The whole page (images and photos) counts as 1 view.
Hit: Is the number of items displayed to your clients including a Web page, graphic or a photo. Each one counts as 1 hit.