The reason isn't immediately apparent as BMP entity beans are not covered in the exam.
Section 12.2.3 of the
EJB 2.0 spec more or less states that the ejbCreate of a BMP bean has to return the primary key object - i.e. it actually returns something. This is when a BMP bean hands over the (new) primary key to the container.
Section 10.5.2 then (in a footnote) states that the ejbCreate method of a CMP bean still has to have a return type of the Primary Key so that a BMP bean can be inherited from a CMP bean.
The container generates the Primary Key based on the settings in the deployment descriptor for a CMP bean. The container hasn't generated the primary key yet when the ejbCreate method runs, because its waiting for you to load the virtual fields from the ejbCreate parameters. So in fact there is nothing you can return - you return "null" instead.