The data might be coming from the .com/.net/.org zonefiles, where a domain will only show up if it has an NS record configured.

I'm not sure if there's a free and authoritative source for registered domains, short of grep'ing zonefiles and checking whois.

There's no other way to bulk lookup quickly besides zone files. Maybe if you are a registrar and even that I am not sure of (depending on how many results you need).