If we are told that a world has dragons, we suspend our disbelief about the dragons specifically and go on considering the show realistic. If a character in that show inexplicably teleports somewhere, we consider that unrealistic because we never expected anyone in the world to have that ability. If a character suddenly kills their best friend, despite talking for ages about how much they love them, we consider that unrealistic because it defies our expectations of how people behave around friends.
