I think it started with them saying that “six million Jews were displaced by WW2” and within a few decades that was changed to “six million killed” because it sold more books and newspapers and sounded way more dramatic. Honestly all it would take is a handful of people saying it on TV without pushback and it would become a “fact” in the culture within a generation. And after that, even if it was wrong, nobody would want to be the guy on TV questioning the numbers.