Imagine the following use case:

1) User comes to your website via the campaign “Emailing November”, via a click on a link in an email tagged with Piwik (or Google Analytics) campaign parameters
2) He browses around and exits after 3 pages
3) He comes back the next day (on 2011-01-01), via a direct entry, and buys a product on your site (which you track as a Goal Conversion)

In this case, if you select the date 2011-01-01 in Piwik, it will show that the Campaign “Emailing November” did not have any visit on this day (because the user entered the website directly), but the conversion is attributed to the Last known referrer that the visitor used (which is stored in the first party cookie). On 2010-12-31, there will be one visit from the campaign “Emailing November” but no conversion on this day.

See also the FAQ: Piwik attributes conversions to the last seen referrer by default. How do I credit conversions to the first referrer instead?.