Holly said in some interview that everyone on the staff called Bob "Barker" and not Bob... she said his FANS call him Bob, everyone else calls him "Barker", so interpret that any way you want. It could be that perhaps "Barker" is more authoritive sounding than Bob, so with Bob's ego, perhaps it's the equivalent of being called "your majesty" or something like that. Also, in Bob's book, he said he was called "Billy" growing up, so maybe he doesn't like being called "Bob" off camera (except by his fans).