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The Goal Overview report displays the source/medium for the converting visits, which may or may not be the same as for the original visits.
A previous post illustrated the principle of last-click attribution: in most reports, including Goal Overview, applies the source/medium of the most recent visit. (The exception is (direct)/(none), which does not overwrite a more specific source/medium from a previous visit.)
This means that Google Analytics would ascribe all credit for a conversion to Google organic if that was the source/medium of the converting visit, even if two visits by that same visitor prior to conversion were driven by Google AdWords and an email campaign.
In an upcoming post, we’ll review the Multi-Channel Funnel reports for a more complete picture of the sources that are generating conversions on your site.