] Does anyone have any ideas as to why this misconception exists? (Please, no spoilers!)

I don't know if an answer can be given without giving spoilers! ;-)

Oh, just thought of something... in Ch. 9, John Thorpe abruptly asks Catherine to confirm that "Old Allen is as rich as a Jew--is not he?" And after establishing that he has no children says: "A famous thing for his next heirs. He is your godfather, is not he?" She denies it. "But you are always very much with them."