How does Flashback choose which photos to include?
Flashback will select a set of photos from the current week in past years. The process is completely automated, and no one views your photos to select what is included in your Flashback.
Why did Flashback show me that photo?
Flashback highlights photos that are more likely to be memories, but—because it's an automated process—not every Flashback will be a perfect presentation of a memory.
If you see a photo in Flashback that you didn't want to relive, you can exclude individual photos or whole folders from your Carousel. That way those photos won't appear in a future Flashback, your Photos, or Carousel.