The title describes the book, literally. This is a diary written by a young girl. No secrets there. Keep in mind that Anne Frank didn’t title her diaries – that part came after her death. At one point in her diary Anne did mention that she wanted to write a book about the experience later, and that "Secret Annex" might be an exciting name for it. The first edition of the book was published as Het Achterhius. Didn’t understand that? Well, it’s Dutch for "Secret Annex," so that went right along with Anne’s intentions. When the diary was eventually published in English, however, the title was changed to Anne Frank: The Diary of a Young Girl.
Neither of these titles, however, gives any hint that the diary is written by a Jewish girl while she and her family were in hiding from the Nazis during World War II. It seems that the publishers, and Anne’s father, Otto Frank, who did the original editing, felt the most important thing about her journal is that it belonged to a young girl. Rather than immediately labeling Anne’s diary as one of a war victim, it is instead presented in a more universal manner, showing that regardless of the circumstances in her life, she was essentially a thirteen year-old girl.