What year was the book published...
A pedophile might lower the age from 17 to 13 to make the reading more tittilating...
Ann may have seemed and acted younger than the average 17 year old of that time...
It is also plausible that Frank Marshall Davis MIGHT be the father if one believes that the events described in his 1968 sex novel, "Sex Rebel: Black", are true and the 13-year old Anne is really 17-year old Ann Dunham.
Frank Marshall Davis