I'm always surprised to hear that a government agency administers polygraph tests in something as serious as hiring but then I remember the CIA also spent millions of dollars trying to develop telekinetic assassins and train clairvoyants to spy on the Kremlin.
The polygraph doesn't have to emit any useful data at all to be very useful in interrogations. Like a bomb doesn't have to have any explosive in it to clear a building. Interrogation is a head game and a complicated box with knobs and buttons and maybe even blinking lights makes a fine prop.
And there's enough ambiguity in it that it's easy for the operator to believe it helps. Like a dowser with their rods, a clergyman with a holy book or an astrologist with a horoscope. That gives them the power boost of sincerity.