Why did the words "horribly good" win the children's interest?

In a low, confidential voice, interrupted at frequent intervals by loud, petulant questionings from her listeners, she [the aunt] began an unenterprising and deplorably uninteresting story about a little girl who was good, and made friends with every one on account of her goodness, and was finally saved from a mad bull by a number of rescuers who admired her moral character.

“Wouldn’t they have saved her if she hadn’t been good?” demanded the bigger of the small girls. It was exactly the question that the bachelor had wanted to ask.

“Well, yes,” admitted the aunt lamely, “but I don’t think they would have run quite so fast to her help if they had not liked her so much.”

“It’s the stupidest story I’ve ever heard,” said the bigger of the small girls, with immense conviction.

“You don’t seem to be a success as a story-teller,” said the bachelor suddenly from his corner.

“It’s a very difficult thing to tell stories that children can both understand and appreciate,” she said stiffly.

“I don’t agree with you,” said the bachelor.

“Once upon a time,” began the bachelor, “there was a little girl called Bertha, who was extraordinarily good.”

“She did all that she was told, she was always truthful, she kept her clothes clean, ate milk puddings as though they were jam tarts, learned her lessons perfectly, and was polite in her manners.”

“Was she pretty?” asked the bigger of the small girls.

“Not as pretty as any of you,” said the bachelor, “but she was horribly good.”

There was a wave of reaction in favour of the story; the word horrible in connection with goodness was a novelty that commended itself. It seemed to introduce a ring of truth that was absent from the aunt’s tales of infant life.