When visitors convert a Goal, Piwik tries to attribute the conversion to the referrer that brought the visit to the site. Piwik by default, will credit the last non empty referrer that was used by the visitor before he reached the Goal. In some cases, several referrers can lead a visitor to the website on different visits (eg. Search engines, Newsletter link).

You can tell Piwik to attribute the Goal conversion to the first referrer used instead of the last one, by editing your Piwik Javascript Tracking code and adding the line below, just before the call to trackPageView()

_paq.push(['setConversionAttributionFirstReferrer', true]);