Poly testing is good, but not infallible. A paranoid schizo and a few other psychos will sail right through.
One of my old bosses would fall straight into that class. He could change his story several times during a ten minute conversation.
Lie detectors also fail if you say what you think is the truth. Someone who was given a good cover story that they believe may be saying something that isn’t true, but the lie detector doesn’t indicate a lie because you think its true.