The book doesn't follow Ove's principles, and he's not above arguing with children about that. Ove's principles dictate that the clown is surely lying and will obviously steal a coin. Though this moment is particularly humorous, it's also sad to see the extent of Ove's principles. Here, they don't help him at all as he both denies the girls a magic trick and doesn't feel that he can trust even a hospital clown, someone whose job is to bring joy.