The first time someone visits a Marketo landing page (or a page on your website that has the Munchkin Tracking Code, Marketo creates an anonymous activity and uses a browser cookie to track it. Once it’s identified, it becomes a person and the history associated with their browser cookie is merged in.
An Anonymous activity is created when someone:
Unlike other links in Marketo emails, View as Web Page is not tracked as an email click.
An anonymous activity is merged into a new or existing person when someone:
One name in the database might be tied to many cookies because people often use different devices and browsers to visit your site.
When anonymous records are merged into a new or existing person record, custom field values will not transfer over.