Sometimes domains have PR left over from sites that may have been on those domains prior to the new owner purchasing them. Or the guy just bought an already established website
yeah, what louis said. Often, domain names are left to drop without being renewed. They can then have up to PR6 in rare cases. However, as these domains drop and are renewed, the PR drops as well, because they are now completely different sites, and their backlinks will be removed in most cases...