Visits are only triggered when a valid request is found for a page, as defined by your PageType setting (or a URL that ends with a slash, which is also considered a page type). Sites however, are counted regardless of the request type. It is very common to have more sites than visits, particularly if you host non-pagetype URLs on your site that are linked to from the outside. If you are not hosting URLs that are linked to from outside sites, then make sure your PageType setting is correct. The default is .htm, .html and .cgi extensions, unless you specify otherwise.
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