This isn't actually true. There may be very few who know how but anyone could be taught how. They're just simple biological tricks to raising your blood pressure, causing sweating, etc. It's true that it's the examiner since the whole thing is an act anyways and the machine is just a prop.

Replace lie-detector in this paragraph with any other pseudo-science like acupuncture or dianetics. "It's not the machine it's the examiner" is a classic way to weasel out of any failure. You can always claim that well, the tester was doing it wrong. If there's no actual method to the process, just the tester's instincts then there's nothing that can be disproved.