You're absolutely correct about the arbitrariness of polygraph interviews. Any operator who tells you they're used to detect truth is flat-out lying. They're

You're absolutely correct about the arbitrariness of polygraph interviews. Any operator who tells you they're used to detect truth is flat-out lying. They're used to put psychological pressure on candidates in order to elicit disclosures. The more honest operators will admit to this. What they won't admit is that they're also used by law enforcement as a smokescreen to obfuscate their terrifyingly inept hiring practices. They work hard to ensure that they maintain a specific departmental culture, and it's very easy for operators to fail a candidate who they think will bring in too much diversity. And I'm not talking about skin color, though I'm sure that happens sometimes too. Basically using polygraphs lets LE continue to hire meatheads. When one of them ends up on the news for acting like a meathead they wave their hands and bluster about "strict hiring practices", and that's the end of it.