There are 2 possibilities the way i see it.
1 - when the whois.sc bot last visited the domain it couldn't reach it for being really not active at that time or unreachable because of a dns problem.
2 - at some cases, if the domain hosts sites at various sub levels but there's no default root page served at the domain's top level, it is interpreted as "not active"
e.g:
www.thedomain.com/a/b/c/page.html exist and active.
www.thedomain.com/ serves no page
hope that helps