If you assume that most or all BJCP judges in training are craft beer enthusiasts and home brewers, accounting for why they suddenly fall way short of their otherwise contemporaries could be explained by understanding how people learn a new skill. Frequently, while assimilating new knowledge, people drop dramatically in ability while they are struggling to get this new info to fit into their world.
I have taught a certain set of psychmotor skills to professionals in my day job. At the beginning of the session, if you have them perform an open skills test, using their existing skill level, they do fairly well. Then teach them several new skills for a few hours. Then go back to the original test, trying to use the new skills. They almost always perform worse than they did before the learning. Thats why its a futile plan to teach too much too soon when it comes to psychomotor skills. Analytically tasting beer is a psychomotor skill. Its hard for new judges to get out of their head. It makes perfect sense to me that a craft beer enthusiast would do fairly well, an experienced home brewery slightly better, and an experienced judge slightly better yet, and the judge in training worst of all. Give them time, they will be at the top.