A former Government Chief of Polygraph, Tom Mauriello, confirms the polygraph is not a lie detector. It is an investigative tool that measures physiological reactions, which polygraph examiners use to interrogate examinees and elicit potential disqualifying admissions. He mentions some agencies will not bring examinees back simply by registering a physiological reaction. Others, recognizing the polygraph is not a lie detector, will bring the examinee back for another polygraph to resolve why those physiological reactions are occurring. In other words, to continue interrogating them for disqualifying admissions. In these YouTube videos, Mauriello explains, alongside some students, a security clearance attorney, and polygraph examiners, what you need to know about the polygraph going in: 

https://www.youtube.com/watch?v=ijnLHqkH1SQ&t=17s&ab_channel=TomMauriello

https://www.youtube.com/watch?v=STCquTdeTD4&t=6s&ab_channel=TomMauriello

https://www.youtube.com/watchv=4V9BFZOrzIk&t=5s&ab_channel=TomMauriello

"Over 90% of all derogatory information that is collected by agencies who use the polygraph...way over 90% of the derogatory information comes from the interview. That means it comes from the mouth of the subject during the interview." 

"The polygraph is not a lie-detector."

"They are showing reactions to certain questions, and we don't know why they are there. The reactions could be because they are not being honest, or it could be some outside issue." 

"The agencies that are willing to bring you back (for another polygraph) are saying 'we recognize the polygraph is not a lie detector; it is an investigative tool that helps us determine whether someone is being truthful or honest.'"