@Thomas,
If someone is willing to take the test and is not particularly sophisticated with regard to how polygraphs work, their willingness indicates a certain sense of confidence that they can tell the truth and not be contradicted by analysis of the machine's results. It doesn't prove anything other than the subject's confidence.
Dr. Ford's credibility was accepted by many people on the basis of her presentation. I think that her polygraph results were secondary — they don't establish her truthfulness but they don't raise questions about it either.
Someone like Kavanaugh, who may be more knowledgeable about polygraphs, may simply believe that the test represents a risk not worth taking. It doesn't prove anything other than his distrust of polygraph results.
I think that when reporters mention her results and Kavanaugh's refusal they are simply adding a few incidental facts to a case which is strikingly devoid of real evidence.
Polygraph tests remind me a bit of dowsing. The devices used and the thinking behind them may be bullshit but the results occasionally conform to reality. And every time a dowser finds water people's belief in the practice is sustained.