There's no limit to our imagination, but there are limits on originality. Describe one modern story to a group of power-readers and you'll get all sorts of "Oh, that reminds me of a book/story by Blah...."
There are several recurring themes out there, love, war, betrayal, redemption, etc. that are recycled endlessly in our storytelling throughout the ages and that does show when the creative ether is flooded with stories of a similar theme. Authors are human beings (hotly debated at times by editors, I know) and they tend to emulate what they like. Don't believe me, watch for the upswing in BDSM themes in the Romance genre now that 50 Shades is out.
Some emulation is even healthy for our storytelling in that it allows for different views, different takes on a theme. An example that comes to mind is "High Noon" a venerable Gary Cooper movie about honor, duty, cowardice, courage and a man facing overwhelming odds to triumph in the end, great themes that resonate through the entire story. Now go watch "Outland" with Sean Connery, a movie that was pitched with the sentence "It's High Noon in space" and produced with that in mind. Same themes, same resonance and another good film.
All it took was some writer looking at something that had already been done and asking "Hey, wouldn't it be cool if...?"