The narrator characterizes Cathy as somewhat incomprehensible. The narrator doesn't understand how Cathy can comprehend things so easily, thing that the narrator herself can't comprehend. For instance, Cathy can understand Granddaddy so thoroughly that she can accurately describe him as being "like a king."
In another instance Granny is telling a recollection about a time when a man was poised to throw himself off a bridge and photographers were there taking photos of him--rather like an attacking chicken hawk circling around a helpless chicken. Granny said that "of course" the photographers left a few shots untaken.
The narrator doesn't understand the implication (the meaning) of Granny's "of course" but Cathy does and proves it by responding with an "of course" of her own. Of course, the meaning of the "of course" is that the photographers were saving film with which to photograph the aftermath of the man's jump....
Cathy is described by the narrator as being only a third cousin and only someone recently added to the family and not close to Granny like the narrator is, since Granny is the narrator's grandmother, not Cathy's. The narrator's implication is that therefore Cathy has no claim to understanding Granny.