Understanding Importing/Exporting

Hello - I have the latest RB, 3.1 i think. I had an existing USB drive with several populated folders. When I clicked said folders and dragged them to the playlist area, it imported the tracks to my collection and began to analyze them. Then "exported" them back to the USB. Some tracks stayed in the actual folders I had set. Other tracks were duplicated into the contents folder with the artist/album/track name structure. NOT ALL FILES Did this. 

Why were some added to the contents folder, and others kept in their local folders?