It appears they are querying this info from refreshed database, and not doing a live reverse nslookup on the server. My understanding is that a reverse nslookup will not provide a list of all domains associated with a particular IP or server.
you cannot trust the accuracy of w/hois.sc
it tracks limited types of domains and is not even good at it
uk domain for instance are not listed at all
the site states clearly what domains they index. if one is looking to include uk domains i'm sure there are lots of sites out there for examples we can share. its less a matter of accuracy and more a matter of limitations.