These websites (or their creators) are likely the only folks able to supply the answer.
Have you looked at the pages underlying html? Do they may direct requests to the registrars (ARIN, RIPE, APNIC)?
These databases in their entirety are available from the registrars.
For some years there was a participant here at Webmaster World that had a website which allowed DNS checks (George's canufly), and he (his site) relayed direct requests to the registrars. As a result of his sites popularity, the registrars place a restriction on the number of requests from a specific site and within a specific time-frame.