I think you have the right to know everything about the exam, but unfortunately, I can't confirm that either way. I suspect that they do go to the UK or Australia for marking by a senior examiner, but nobody knows for sure. You certainly can't be too paranoid about these people- they are greedy, secretive and wholly unethical.
However, I believe that most of the problems can be explained by something much less interesting than the conspiracy theories flying around (which, may in fact be true, we just have no convincing evidence either way)- the lack of training and monitoring of examiners, and their tendency to mark low rather than high when uncertain, is the most logical explanation, and seems to explain the situation fairly well, compared to the other things people think. Again, I can't prove this, but most of the other explanations lack something, or clearly rely on 'facts' which just don't make sense.
If we consider that 'usually' the result are 'about as expected', then the remainder is easily explained by lack of examiner accuracy.
However, when you convince multiple governments that your exam IS well monitored and reliable, you charge $300+ for an exam, and people's futures depend on your lies, then 'usually' and 'about as expected' are completely unacceptable.
The exam sucks, and the people who make it are a kind of mafia. However, I just don't believe that the explanations you hear for how this happens are convincing. And think of the result- more people giving these criminals more of their money for remarks, only to be disappointed yet again. It's shameful beyond belief, but if you act according to unfounded rumour, then in a way you are part of the problem.
