Polygraphs are junk science. I wonder why they haven’t graduated to fMRI. Can’t be for lack of funds. My guess is the polygraph bureaucracy is what’s known in Washington as a self-licking ice cream cone.
Perhaps the point is it's "confession theatre". You're put in a stress position, worried that the "magical machine" can read your darkest secrets, and told that everything will go easier if you're just honest, and so that's why you're inclined to spill them. Which is what they are trying to get you to do.