Now that we’ve cleared up the myth, one question remains, at least for me: Where did it come from? And why, out of all time spans, 7 years?

Someone has sometimes read the real research about... Let's say blood
He forgot what he read. But needed some urgent, late night, 500 words text for newspapers to make the quota for the day. He wrote something.

Without the Source (because - daily newspapers)

It perpetuated for 20 years or more. There should be a law about publishing without the source.