Many times they will make comments to get a reaction from you, such as:
"Looks like you are telling the truth.." (even if it shows otherwise)
If you look relieved or surprised, this might be something that they can use at a later date or further along in the interview. Polygraphs are 100% interpretation, they are not binary by any means.
Exactly, it's just an interrogation prop like when the East German police would have the fake telephone where they pretended to make calls to family/friends and verify your story, and had a button under their desk to make the phone ring so they could pretend to get just in time information to refute your story. Modern police will go into an interrogation with a decoy file folder full of random papers and then go through it claiming the contents refute your entire story and you'd better confess too.
A standard polygraph trick is to switch it off, and then have a casual interview during the middle of the test. Then they claim they never switched it off and you've been lying the whole time as your casual results differ on the magic box. Props tend to work to get people to trip up on their replies even if the prop is just a box that randomly scribbles on paper with absolutely zero scientific credibility.
"Looks like you are telling the truth.." (even if it shows otherwise)
If you look relieved or surprised, this might be something that they can use at a later date or further along in the interview. Polygraphs are 100% interpretation, they are not binary by any means.