Bizarre?

A few weeks ago I posted an article about a rare and quite frankly silly sounding psychological disorder called Cotard's syndrome. In which a person holds a delusional belief that he or she is dead, does not exist, is putrefying or has lost their blood or internal organs. Interested by the absurdity of these delusions I continued to investigate other delusional disorders. Interestingly, I found that although there are countless causes for delusional disorders, commonly there are only two categories of delusions—bizarre, and non-bizarre. Non-bizarre delusions are considered to be plausible; that is, there is a possibility that what the person believes to be true could actually occur a small proportion of the time. Bizarre delusions, focus on matters that would be impossible in reality i.e. Cotard’s syndrome. What I don’t understand however, is why this seemingly non-scientific and subjective distinction between bizarre and non-bizarre delusions is used to treat different people differently. It would seem, that what is considered bizarre would depend on social norms. For example, why is one man considered delusional when he thinks the government is watching him at all times, and the person treating him as if he is crazy believes that there is a greater being looking down on all of us form a non-existent place. To me the latter of the two seems less possible than the former. Perhaps this is the reason why there is far more research in to mood disorders then in delusional disorders. Who is allowed to determine the ultimate reality? Why do some people think that they know what is possible and what is not? Is it not possible that we are all delusional in our own respects, and that it is our culture that determines weather or not we are bizarre?