I think this is a minor corollary of Murphy's Law

Sometimes, no matter how accurate a historical novelist strives to be, she just cannot track down some detail or other. Such as, say, any kind of description of the appearance or character of a minor historical figure. Eventually, the aspiring novelist decides to make it up, or at least to extrapolate based on the thin evidence she can read between the lines when he makes an appearance in the biographies of the more prominent.

"Hm," thinks the writer. "He seems to be the one everyone else spoke to, even when they weren't speaking to each other. He had a long and apparently successful career as an ambassador, which would imply that he was, well, diplomatic. And he was the youngest of a large family. I bet he was mellow and easygoing."

Satisfied with her deduction, the writer pens two chapters wherein this mellow, unflappable man plays a large part. Throughout his scenes, our boy is calm and capable.

A few days later our writer stumbles across a line or two describing the gentleman in question in a biography of one of his more prominent relations. That calm, unflappable man? Actually, the family drama queen.

Well, I already knew I'd have some serious rewriting to do once NaNoWriMo is over...