Sitefinity does not use directly the XML element names but the names of the properties of the SyndicationItem type. So Summary in the mappings refers to the property of the type. In RSS the Summary property of a SyndicationItem is rendered in the "description" element. In Atom protocol the Summary property is rendered in the "summary" element. Inside the publishing system of Sitefinity if no Content of the syndication item is provided than the value of the Summary property is used for content.
As a solution we suggest using the Atom protocol in your feed. Please let us know if that solution will work for you.
All the best,
the Telerik team