In Hard Times, why does Mr. Gradgrind insist on calling Sissy Jupe by the name Cecilia?

Mr. Gradgrind does not consider Jupe a name,

“Sissy Jupe, sir,” explained number twenty, blushing, standing up and curtsying. “Sissy is not a name,” said Mr Gradgrind. “Don't call yourself Sissy. Call yourself Cecilia.