Where the list comes from
Every publicly trusted TLS certificate is recorded in Certificate Transparency logs, open append-only ledgers that browsers require. The hostnames inside each certificate are therefore public. OrbitProbe searches those logs through crt.sh and collects the names that fall under the domain you entered.
That makes the list a record of certificates, not of DNS. A name can be listed and no longer exist; a name that only ever used a wildcard certificate, or none, will be missing. Wildcard entries such as *.example.com are shown separately because they cover names without revealing them.