If a potential customer comes to your website for the first time, or if he visits a landing page more than 30 minutes after his last page visit, this will be recorded as a new visit. A Unique Visitor can make multiple Visits per day.

It can happen that more visitors than ad clicks happen in one flight. The reason for this is usually either the technical definition of a visit (see above) or users (i.e. from your own organization) who know the URL and click on the published landing page after the flight was published.