The polygraph machine is, of course, mostly just a prop. Although I think perhaps some operators believe in it, there's actually no evidence to show that of the things it measures can with any reliability determine whether a person is telling the truth or lying.
It's not a 'test', it's an interrogation. They are trying to convince you that they know more than they actually do to try and stress you and push you into making a confession.
I’ve heard it suggested that the point of the polygraph isn’t to serve as a prop or to discern truth from falsehood, but rather to direct the interrogation toward topics that seem to cause the interviewee the most stress.
It's not a 'test', it's an interrogation. They are trying to convince you that they know more than they actually do to try and stress you and push you into making a confession.