for .ac has a record, unlike, for example. zone .EN. .io, which is a record. Perhaps because they are relatively fresh. Why so done — we can only guess.
# dig ac
\r
; <<>> DiG 9.3.1 <<>> ac
;; global options: printcmd
;; Got answer:
;; ->>HEADER<< — opcode: QUERY, status: NOERROR, id: 58178
;; flags: qr rd ra; QUERY: 1, ANSWER: 1, AUTHORITY: 7, ADDITIONAL: 0
\r
;; QUESTION SECTION:
;ac. IN A
\r
;; ANSWER SECTION:
ac. 85584 IN A 193.223.78.210
\r
;; AUTHORITY SECTION:
ac. 85584 IN NS b.ns13.net.
ac. 85584 IN NS b.nic.io.
ac. 85584 IN NS b.nic.ac.
ac. 85584 IN NS a.nic.ac.
ac. 85584 IN NS ns3.icb.co.uk.
ac. 85584 IN NS ns1.communitydns.net.
ac. 85584 IN NS a.ns13.net.