When the headline is contradicted by the article then the relevant quite is relevant and valuable. It saves time. Not everybody reads the article, I'd say that it even is rare and that a lot, maybe even most people jump to the discussion immediately after reading the headline. Because of cases such as this one: When you quickly see the headline being contradicted.
Usually articles are a lot longer and they bury such info deep, so checking the comments first if its even worth clicking on the article is normal, given the huge number of sensationalized headlines coupled with the attempt of the article writers to keep readers on their page for as long as possible instead of coming to the point right away.
The reading behavior of many is optimized for sifting through a large number of submitted headlines without wasting too much time, not for maximum enjoyment of any single one of them. The (I think reasonable) assumption is that many, maybe most, even of the initially interesting sounding headlines turn out not to be worth reading.