Well, 'Winn-Dixie' is often classified as fiction, but it might have elements that are reminiscent of real life. Maybe the author was influenced by real events or people to create the story, but it's not a direct account of something that happened exactly as written.
In Supergirl fanfiction, a pregnancy involving Winn and Kara can be a very interesting plot device. It could bring out different aspects of their characters. For example, Winn might show his more protective side. However, in the official storylines, there's no such plot, so fanfiction gives writers the freedom to explore this uncharted territory.
It can be classified as historical fiction. The story often references and reflects the social and cultural aspects of a particular historical period, giving it that genre classification.