a history of a person's life written or told by that person
Because she kept a diary and it was published. The only reason anyone is the subject of an autobiography is because they wrote about their own life.
The first president to write an official autobiography was Ulysses S. Grant. He is followed in this by fourteen other presidents, including Theodore Roosevelt, Calvin Coolidge, Herbert Hoover, Harry S. Truman, Dwight Eisenhower, Lyndon B. Johnson, Richard Nixon, Gerald R. Ford, Jimmy Carter, Bill Clinton, and Barack Obama.