The "hard science" behind all these stories is that no matter what the story actually says - suspected weapons were found, were not found, are still being looked for, might be there, HERE THEY ARE!, oops, no they aren't, etc etc etc etc etc., they all build up a cognitive association between "Iraq / Saddam Hussein" and "chemical weapons". When you think of one, you think of the other.
Even if each story mentioning a supposed 'find' is then followed by another story taking the first story back, this association gets built up. Even if the reader reads all these stories, and the net information in the pile of stories is that 15 accusations were made but they were all proven to be lies, mistakes, unfounded, etc., the reader is left with the association and the belief that the weapons are there somewhere.
There is a whole literature on this sort of thing. These people are not fools. The beauty part is that the journalists can say, "Well, you see the system works, we reported when the claims were made and we reported when they were disproved." And yet the effect is there. It's not easy to beat this.
LP