It's the first impression that mattered, I guess. The movie started with Zeena caring for Ethan's mother, and then showed Ethan's emotional dependency on her after his mother died. But the book started with heavily painting the loveliness of Mattie and Ethan's geniune feelings toward her.
Like in The Age of Innocense, Wharton never clarified what had been exactly in the wife's mind, which left space for the readers' imagination.