This passage also shows that Catherine is not used to Mr. Tilney's manners, and perhaps, does not fully approve them.

Her idea of a gentleman was likely formed by her father, who is described as 'respectable,' and likely is plain-spoken, and sensible. Catherine does not trust her own judgment enough to act on it, perhaps by mentioning Mr. Tilney's lack of respect for an elder when he questions her, but the discomfort is there.