Yap it is just an interrogation. However, knowing about how the polygraph is ineffective and letting the administrator know will often result in 'FAILURE'. They will specifically ask questions related to polygraph terminology (lingo, abbreviations) to see if you are familiar.
It effectively lets through psychopaths, very good liar or those that know how polygraphs work and presumably practiced passing it.
Of course it's also a great medium to screw someone over, but that's a whole other thing.