HN lets reposts through if the story hasn't had significant attention yet. This is to give good submissions multiple chances at getting attention. Past explanations:
I know it sucks when your submission was earlier and gets overlooked! We should eventually have some sort of karma-sharing to take care of this. In the meantime, it at least evens out in the long run, since the reason is randomness.
Curious why HN didn't merge the submission as it usually does. Is there a "no, submit this again" option?