I'm currently enrolled in the CraftBrewers Apprenticeship through the American Brewers Guild, and just went through a whole lecture on sensory evaluation. It was some pretty interesting stuff, about the difference between sensory analysts (trained evaluators) versus organoleptic tasters (untrained, emotional tasters) and how they attempted to quantify the evaluations as much as possible. They also talked about sensory biases and how they affected the evaluations.
In short, there's a WHOLE LOT of factors that can influence beer judging- many of which are outside the control of the brewer.