Within a Smart List, there are two filters: 'Person was Created' and 'Created At'; both similarly named but qualifying different records.
The 'Created At' filter is looking at the date of when the Marketo record was first created. This date is listed under the Info section on the person record.
An individual could visit a Marketo page for the first time and generate an initial web page visit on May 1st tracked on an anonymous record in the database. Then on May 8th the person fills out a form thereby converting their anonymous record to a known record in the database.
May 1st is the date used for 'Created At' filter, and May 8th is the date used for 'Person was Created' filter.