Another thread brought this to mind.
Back when I was trying to sell my stories, I submitted "Little Swimmers." They found another version, and questioned my authorship. (Weirdly, neither they nor I found the copy I had on SOL. I removed it much later.)
The other version took more than half my story word-for-word, and then fit it into the "author's" standard story-frame. My story, however, was of a married couple deciding to start a baby; the guy's standard story-frame involved uncle-niece incest.
So the stolen parts fit very poorly. If the guy could write the other half, why couldn't he write the entire thing?