Through all the research I've done after failing my first poly, I've come to the conclusion they're voodoo nonsense. I've passed CVSA and polys since my dreaded first one, and gave the same answers.
After finding out everyone in my applicant group at that department failed, I've become suspicious that those involved in the poly phase for that department have no business administering them. At least one applicant I figured would be a shoe-in due to already being in that particular field and seemed respectable enough.
I understand under certain conditions they're a useful tool, mostly to convince those uneducated on how they really work into fessing up. But they generally seem to screw honest people over.