I think it might be something like this:
There is a population of people who truly believe in bigfoot. Let's call them A.
There is a smaller population of people who claim to have seen a bigfoot. Let's call them B.
There are some members of population A in population B but B is not a strict subset of population A. It is, in fact, dominated by population C which is comprised of people who claim to have seen a bigfoot but do not actually believe in bigfoot.
Population C is disproportionately important in bigfootery because these people not only make up the majority of the claims, but they are the ones pushing the growth of population A. This includes your Pattersons, Biscardis, Meldrums, Byrnes, etc.