How does the doctors interpretation of mrs mallards death differ from what actually happened?

In "The Story of an Hour" by Kate Chopin, the doctor's proclamation that Mrs. Mallard has died of "the joy that kills" is ironic, because the reader knows that it is the exact opposite. Mrs. Mallard was actually feeling free and excited about her new life after the death of her husband, and finding that he was still alive was shocking because she was robbed of that new life.