I am in no way an expert in polygraphs, but having gone through the whole polygraph issue with Hailey Dunn's case, it seems to me that you pass/fail (or show a physiological indicator) for each question. Therefore, it would be logical to conclude that they may have failed a question or two (e.g., what if the little boy wasn't riding with Breeann and she was on her own but they didn't want LE to know that -- just speculation on my part for the sake of argument here, or, what if the timing was off and both parents answered differently to a few questions, but truthfully for how they remembered it, etc.).
I think it's likely that they (or anyone) could miss a few questions, or it's also possible that LE told them they failed and they didn't. While not a "nice" thing to do to innocent people, I personally give LE a pass to do this if they think someone is holding back.
All of this is JMHO... Again, I know nothing about LDTs.