Why does Harvey give different names for his wife?

Mr. Harvey knows he can appear odd and out of place. Because of this, he always has a sympathetic story at the ready in case he is questioned by a suspicious person or the police. In order to come up with a convincing story, he uses his most recent victim to help him create a fictitious wife. He thinks about her and speaks reverently. This demeanor and attention to detail makes him appear harmless, although obviously he is not.
Mr. Harvey changes his wife's name because he creates a better character with the victim who is most recent in his mind. This backfires on him when Mr. Salmon notices that he has given two different names for his wife. Unfortunately, this does not help the police catch him as they dismiss Mr. Salmon's concerns.