That's the thing, they want to know if you would bring up things even if you didn't think they knew about them. They want people who would do that, due to believing the polygraph works.

So they questioned them about this thing they knew about and he didn't reveal it, so they thought he was lying even though he honestly didn't remember it.

It doesn't work out well for people who know how polygraphs work in general or for people who do not reveal things they know about (intentionally or otherwise).