The pinboardr package returns the data in a format as close as possible to the data supplied by the API. Because 'pinboard.in' modeled their API on the original 'Delicious API' with the same unfortunate choices. See also the description on pinboard posts_add docs.
I try to hide the complexity from you by translating but sometimes that gives us different names. I first give you the pinboardr name and than the name in the delicious and pinboard API:
The 'title' of a bookmark is actually called 'description'.
The 'description' of a bookmark is actually called 'extended'.
The 'public' setting of bookmarks is called 'shared'.