why is the robbery conversion in chapter 13 ironic?

Justice Robinson, John Buckstone, and Constable Blake stop by Wilson's house to discuss the recent thefts. The constable reveals that a woman is suspected of the robberies. Wilson immediately thinks of the mysterious young girl he had seen in Tom's room, but she is quickly put out of his mind when the constable specifies that the suspect is a stooping old woman that someone saw coming out of one of the robbed homes and the constable himself saw getting onto a ferry. Wilson notes that the old woman will have difficulty pawning off the items she stole, as the twins notified the pawnbrokers when they discovered the Indian knife was missing. Luigi and Angelo have offered a five hundred dollar reward for the weapon, as well as a reward of equal value for the thief.