Do you think a prophecy takes into account its own existence in the future when prophecies are made? Like, are they reporting on a future that would still happen without them, or is the future they're showing seers one that's entirely contingent on their existence being the catalyst? If it's the latter, then it seems likely that the prophecy about Harry/Neville and Voldemort was the seer-magic's way of stopping Voldemort from rising to power unchallenged, so it created itself in order to give those involved the tools to stop him. So either magic is biased, or Voldemort's reign would have had really, really bad consequences for everyone.

Although, I guess most of them are probably never heard by the people involved, but then you have to wonder if those prophecies knew that when they were made, and if that's true, then why make themselves to begin with? So many questions... I wish JKR would write more books about different great witches and wizards where she fleshes out the magical theory and lore of the HP world. Fanfics try but they're hit or miss.