The writer, and editor of the journal of concise nonfiction, Brevity, was interviewed by Mary Richert as part of her nonfictionist series on her blog No Titles:
I think certain experiments, with language, point-of-view, structure, work better in the short form. Very brief essays are like a petri dish for innovation.
. . . [T]he lyric, almost ethereal essay as opposed to the highly journalistic ‘article’ – are both nonfiction, and nonfiction that allows creative choices on the part of the author . . . But fiction has this range as well: fiction includes the child’s board book and the highly formulaic western, and all of the literary genres and sub-genres, including the works of Gabriel Garcia Marquez and Italo Calvino.
I think any story can be set as fiction or nonfiction (though of course, the nonfiction story must be limited to the honest facts, memories, and observations, and the fiction author can , and should, allow the imagination to add and enhance). Maybe the difference is with the author — some authors need to explore an experience, idea, or question in one mode while others need to explore it in a different mode.