dear Bob, as an 'rfog' (where in heaven did scott come up with that) i think as well as the other reasons the other chaps have given, sometimes the briar will have slight variations and may not be so mature or from a diffrent batch,a 'rofb' or a 'rnfb' .
you'll find some smoke sweet others don't, it's the luck of the draw.
great title Bob for the post, you really make me feel at home.