We hear the following kind of statement from our SCJP trainees many times, "...I didn't think they would ask about it because it wasn't mentioned in the objectives, but I glad I studied it anyway." This could be because of the reason that Bert mentioned or because of something else.
Another issue is that every topic does not necessarily have a clearly defined area. It may unintentionally touch upon some other topic that has not been explicitly mentioned in the objectives.
Regardless, we believe that there is no harm in reading/learning a few extra things that seemingly are beyond official exam objectives. Because no matter whose fault it is, it is the candidate who loses the marks