Why does the narrator's statement, "The boy's name is Dickie, not Tickie" (p. 39), have such an overwhelming impact on Ha'penny?

The young street urchin Ha'Penny has constructed a whole backstory for himself. He's told the narrator, his social worker, that he comes from a large family consisting of a mother, one Betty Maarman, and four siblings by the name of Richard, Tickie, Anna, and Maria. As the narrator tells us, he's never dealt with a street kid before who has such a strong desire for a family. But if that's the case, he asks Ha'Penny, why did he steal from his mother?
As the narrator probes Ha'Penny more deeply about his alleged family, it's clear that the young waif has been making it all up. Such a family does indeed exist, but he's not a part of it. Ha'Penny's phony story is finally rumbled when the narrator points out to him that the boy he claims is his brother is called "Dickie," not "Tickie."
At that moment, all of Ha'Penny's brave assurance suddenly vanishes from his face. Now he stands humiliated, a vulnerable child whose every last vestige of pride has just been destroyed with the exposure of his fictitious backstory. Ha'Penny's tall tale had given him a sense of human significance, a sense of belonging. But now that the story's been revealed as nothing but a tissue of lies, that's all gone out of the window. Without his self-constructed identity, poor young Ha'Penny now feels utterly worthless.