LE might tell the public/media some suspects/POIs passed but saying that publicly on purpose so POIs can rest on their laurels, feel confident they got away with it, create a sense of security and maybe slip up.
There is debate whether LE can 'lie' about the results of polygraph for strategic purposes, to get a confession or get someone to rat, etc.
Interestingly, polygraphs are being discussed in another (Canadian crime) thread I'm in. I'm going to paste below what I put in that one a few hours ago:
Yes, they can be used in the investigation steps but as a tool - so I guess to me that is still a bit like baiting or bluffing. Since it's not really tangible and cannot be used as evidence nor admissible in court. I do think it has be used to manipulate, bait, bluff, spook, unsettle a POI, suspect, associates/family/friends, etc.
I think investigators know how to use this tool to their advantage and very effectively whether it's privately in the police station interviewing someone connected to a crime or mentioning it to the media strategically as part of their investigation.
MOO JMO
*ETA: 3 cases I'm currently following in Canada that are unsolved, LE said all suspects/POIs in all of them passed polygraphs. I definitely take it with a grain of salt as I'm almost certain in 1 or 2 of them the person(s) are involved guilty!