In the novel That Was Then, This Is Now by S. E. Hinton, what did Byron find surprising about Cathy's personality?

In Chapter 3, Bryon takes Cathy to a school dance, where he runs into his ex-girlfriend, Angela Shepard. Angela approaches Bryon and he dismisses her by asking if she came with Ponyboy Curtis. Angela responds to Bryon's comment by cursing at him while she walks away. When Angela leaves, Cathy asks Bryon about Angela, and he tells Cathy that Angela was someone he used to date. Cathy then makes a sarcastic comment about Angela's "nice language" and refers to her as a "real lady." Bryon is shocked at Cathy's response because for the first time he realizes that Cathy wasn't jealous of Angela at all. Bryon mentions that Cathy was the first girl he's ever dated who wasn't scared that some other girl would show her up. Initially, Bryon doesn't know what to make of it but then decides that Cathy has a lot more sense than most girls.