Good points all, but, at its core, its "judging" - using one's best judgment about a subject, according to some set of criteria (in this case, BJCP guidelines). It has to do with the Judges' perception of the beer, not its chemical makeup as determined by some sort of lab testing. The guidelines are just that - guidelines, not definitions. How you get to a brew is up to you, but a brew made according to the guidelines are more likely to be perceived by a trained judge as deserving of a good score. And judges are human; error prone to some degree and beset by the vagaries of taste, perception and preference, no matter how much they may try to be totally objective. And my hat is off to them.
Its a flawed system, but the best around.