You really can't get a 35 then two weeks later get 21 if everything is the same. The system is better than that. I would assume that even though it was the same batch of beers, the bottle they tasted from at one event did not contain the exact same thing as the bottle from the other event.
About 80% of the points in any BJCP sanctioned judging come from very objective criteria, like color, carbonation levels, ABV, clarity, the presence of style factors (like a sour must be sour, an IPA must have a certain level of hop bitterness and aroma, etc.), so there really isn't any sort of conspiracy or even room for interpretation for the exact same beer to be judged differently on the majority of points.
That said, the remaining 20% are a bit open to each invidicual judge's interpretation. People's palates are different, and a fruity, floral flavoring hop to one judge can easily seem a bit soapy or medicinal to another. That's why every beer is always judged by 2-3 judges, and if their individual scores vary by 7 or more, the BJCP guidelines urge the judges to discuss why they are apart and compromise to within a 7 point spread.
The vast majority of judges I have met take the job very seriously, and they are always trying really hard to give points, not deduct them, but take pride in knowing the style guidelines and sticking to them as strictly as possible within their experience.
Where brewers get all twisted is when they are ignorant of the style guidelines and think they should get good scores just based on how good the beer is. It literally has to tick all of the style points to score highly. Just because the recipe you bought is named "American IPA" and you brew it perfectly doesn't mean all of the criteria land firmly in the BJCP criteria. You need to know the guidelines and make sure everything in the recipe is in range with the criteria BEFORE you brew it, then double check afterwards.
If you KNOW the criteria, you can pretty much figure out what your score will be before submitting it, within a reasonable range. You are not going to brew a beer that legitimately lands firmly in the middle of the criteria and get a 20. There is no conspiracy. The system's as professional and objective as anyone could hope for.
I had a beer over the weekend that was an absolutely phenomenal Baltic Porter. Higher ABV balanced by sweet malt with licorice and black currant notes with a clean, lagered finish. However, they entered the beer under Robust Porter. Very different beers with very different style criteria. It may have scored a 40+ in the Baltic Porter category, but scored about a 30 in the Robust Porter category. Not a value judgement on the beer itself, just how it measured against the established criteria for the category entered. The brewer may have literally just made a mistake in the category entered, or may just not have done his homework on the style criteria.
Either way, I not only judged it properly against the Robust Porter criteria, but at the end told the brewer that he should review the guidelines and consider entering it again as a Baltic Porter. If he follows the advise, I think he has a great recipe that would score well.