They don't monitor the domains for every change, like the SnapNames/Dialog "WHOWAS" service attempts to do.
Instead, WHOIS.sc is simply storing the WHOIS data everytime someone does a search at
www.whois.sc for the domain. So, if someone never checked the WHOIS for your domain on a certain date, it won't be saved.
http://openaccess.dialog.com/ip/
is the DIALOG alternative, in partnership with SnapNames.