I would agree that newer Judges are sometimes overly critical at first, and I've seen some cooks in the judges tent that have been too. That said, I've judged with some experienced cooks that were very interesting to visit with. The JCUP training put a lot of emphasis on the variability point, and gave pretty good examples. One for example is the fact that KCBS scoring has no provision for uniformity or focusing on imperfections which can easily come into play in both chicken and ribs. If you have ever read scores/comments on the BBQ Critic Judge My Box site, some are very harsh if samples lack uniformity. You get the impression some commenters spend way more time that 5 or 6 seconds looking at the box photographs.
Have you looked at your past scores on the KCBS site? On the high scoring entries, the scores are usually within a point of the table average. But when an entry really bombs, it's easy to see that some Judges don't go lower than a 7. I suspect it's to avoid filing out a comment card, but it's not fair to the cooks if something deserves a 6.
And speaking of a 6 which is now "Fair", examples of scoring a 6 are: samples that are overcooked, undercooked, too spicy, or too bland. A 5 (poor) may be appropriate if a sample is too chewy, dry, burnt, mushy or the taste/tenderness has so little appeal you would not want another bite.