Last year I had a porter that got beat to hell at the OC County Fair... it then took a Gold Medal at the LA County Fair. It got a decent score at another comp as well. Same batch, bottled on the same day. I have to assume that either the OC County Fair got a bad bottle or there was a mix-up. Honestly, the feedback was so far off the others and what my wife and I could detect in the beer that we still believe there was a bottle mix-up during the judging at the OC County Fair. LOL
This was the most extreme case of differences, but I've seen many other small cases of it. I try to enter 3 or 4 comps with the same beer to get a good evaluation.