Should I learn more than what's in the specification?

I know this may sound stupid, but basically...
I am currently making notes for Add. Science and I look at both the textbook and the specification. Sometimes though, my revision guide adds stuff that isn't in the specification, so my question basically is: Is it better to either only remember and learn what's in the specification really well (and skip over anything that isn't included) or is it better to try and learn the additional content in the revision guide as well as the main points in the specification?