I've never entered a competition nor judged one but I suspect it can be rather subjective. I went to a beer tasting and one of the samples was a coffee IPA. I hated it, one sip and pour it out type of hated it. I thought it tasted of extremely strong, stale, cold coffee as did 4 others at my table. The other member of my table loved it and just couldn't get enough of it. Different tastes.
Well, in competition, the judges sit with the BJCP style guidelines in front of them (or on their iphone app!) and judge 100% by the style guidelines. While taste is always subjective, the judges are trained to recognize off-flavors, and know what each style should taste like.
None of us who are judging are even told if it's an extract beer or AG beer. We just taste the beer, and judge by aroma, appearance, flavor, mouthfeel, and overall impression, according to the style guidelines. If there are any off-flavors that shouldnt' be there like acetaldehyde, mustic, metallic, etc, you will get dinged for that. Some flavors like DMS, phenols, esters and diaceytl are perfectly acceptable in certain styles but out of place in others.
Sorry to go off-topic here, but saying the beer competitions are subjective got my dander up. Judges go to "judge school" and work hard to study for the bjcp testing, and do it without pay or reward. Judging competitions isn't easy- and you get PLENTY of bad beer. Trying to make constructive remarks to fix a problem takes time and energy, and all of the BJCP judges I know do their best to do that. You won't get a "this beer sucks" scoresheet back. Scoresheets take a long time to fill out properly, and you will get constructive feedback as well as some possible fixes like "this beer has a phenolic character that is not expected in a blonde. Perhaps checking fermentation temperature and/or chlorine in the brewing water would be warraranted", etc.
Anyway, a good beer is a good beer whether made with grain or extract.