Because no one tells Eleanor what they are thinking, she unwittingly provides more "evidence" in her defense of Mr. Slope. Based on what her family actually says, the conflict lies with Mr. Slope's religious views and that is what Eleanor addresses. She does not understand that her family is really talking about a romantic relationship because her family members rely on innuendo rather than direct tactics. This repeats itself in the inability of Eleanor and Mr. Arabin to express their feelings directly. Instead they rely on metaphor and assumptions to get the other to understand their feelings.
Barchester Towers