IO apears to have nameservers operated by Afilias, CommunityDNS, OneWorld DNS (which appear to also be CommunityDNS)

$ for ns in $(dig +short io. IN NS | sort); do
ip=$(dig +short ${ns} IN A);
rev=$(host ${ip} | awk -Fpointer '{print $2}');
echo -n "= $ns = $ip = $rev = ";
whois -h whois.ripe.net ${ip} | awk -F: '/^descr:/ && !/RIPE/ {print $2}';
done

= a0.nic.io. = 65.22.160.17 = a0.nic.payu. = Afilias NS087 A0
= b0.nic.io. = 65.22.161.17 = b0.nic.payu. = Afilias NS087 B0
= c0.nic.io. = 65.22.162.17 = c0.nic.payu. = Afilias NS087 C0
= ns-a1.io. = 194.0.1.1 = ns1.communitydns.net. = ICB plc Anycast (via NetConnex AS21396), ICB plc Anycast (via Community DNS AS42909)
= ns-a2.io. = 194.0.2.1 = ns-a2.io. = ICB plc Anycast (via Community DNS AS42909)
= ns-a3.io. = 74.116.178.1 = b3-1.oneworlddns.net. = Community DNS
= ns-a4.io. = 74.116.179.1 = b4-1.oneworlddns.net. = Community DNS

It looks like a couple of the CommunityDNS nameservers are carrying a different version of the zone than the others:

dkmpb:~ dknight$ dig -4 +nss io.
SOA a0.nic.io. noc.afilias-nst.info. 1497256293 10800 3600 2764800 900 from server 65.22.162.17 in 38 ms.
SOA a0.nic.io. noc.afilias-nst.info. 1497256292 10800 3600 2764800 900 from server 65.22.160.17 in 44 ms.
SOA a0.nic.io. noc.afilias-nst.info. 1497256201 10800 3600 2764800 900 from server 194.0.2.1 in 62 ms.
SOA a0.nic.io. noc.afilias-nst.info. 1497256293 10800 3600 2764800 900 from server 194.0.1.1 in 64 ms.
SOA a0.nic.io. noc.afilias-nst.info. 1497256293 10800 3600 2764800 900 from server 65.22.161.17 in 98 ms.
SOA a0.nic.io. noc.afilias-nst.info. 1497256293 10800 3600 2764800 900 from server 74.116.178.1 in 101 ms.
SOA a0.nic.io. noc.afilias-nst.info. 1497256201 10800 3600 2764800 900 from server 74.116.179.1 in 107 ms.

And a couple of them are giving bad response

echo = Afilias
dig +noall +norecur +cmd +answer +authority -4 +notcp @a0.nic.io. io. IN SOA hostname.bind. CH TXT shl.io. IN NS
dig +noall +norecur +cmd +answer +authority -4 +notcp @b0.nic.io. io. IN SOA hostname.bind. CH TXT shl.io. IN NS
dig +noall +norecur +cmd +answer +authority -4 +notcp @c0.nic.io. io. IN SOA hostname.bind. CH TXT shl.io. IN NS
echo
echo = Community DNS
dig +noall +norecur +cmd +answer +authority -4 +notcp @ns-a1.io. io. IN SOA hostname.bind. CH TXT shl.io. IN NS
dig +noall +norecur +cmd +answer +authority -4 +notcp @ns-a2.io. io. IN SOA hostname.bind. CH TXT shl.io. IN NS
dig +noall +norecur +cmd +answer +authority -4 +notcp @ns-a3.io. io. IN SOA hostname.bind. CH TXT shl.io. IN NS
dig +noall +norecur +cmd +answer +authority -4 +notcp @ns-a4.io. io. IN SOA hostname.bind. CH TXT shl.io. IN NS

====== Afilias ======

; <<>> DiG 9.8.3-P1 <<>> +noall +norecur +cmd +answer +authority -4 +notcp @a0.nic.io. io. IN SOA hostname.bind. CH TXT shl.io. IN NS
; (1 server found)
;; global options: +cmd
io.	86400	IN	SOA	a0.nic.io. noc.afilias-nst.info. 1497256290 10800 3600 2764800 900
io.	86400	IN	NS	ns-a3.io.
io.	86400	IN	NS	b0.nic.io.
io.	86400	IN	NS	ns-a1.io.
io.	86400	IN	NS	ns-a4.io.
io.	86400	IN	NS	ns-a2.io.
io.	86400	IN	NS	a0.nic.io.
io.	86400	IN	NS	c0.nic.io.
hostname.bind.	0	CH	TXT	"ns087b.app22.iad1.afilias-nst.info"
hostname.bind.	0	CH	NS	hostname.bind.
shl.io.	86400	IN	NS	ns2.p23.dynect.net.
shl.io.	86400	IN	NS	ns1.p23.dynect.net.
shl.io.	86400	IN	NS	ns3.p23.dynect.net.
shl.io.	86400	IN	NS	ns4.p23.dynect.net.
shl.io.	86400	IN	NS	ns1.d-zone.ca.
shl.io.	86400	IN	NS	ns2.d-zone.ca.

; <<>> DiG 9.8.3-P1 <<>> +noall +norecur +cmd +answer +authority -4 +notcp @b0.nic.io. io. IN SOA hostname.bind. CH TXT shl.io. IN NS
; (1 server found)
;; global options: +cmd
io.	86400	IN	SOA	a0.nic.io. noc.afilias-nst.info. 1497256290 10800 3600 2764800 900
io.	86400	IN	NS	ns-a1.io.
io.	86400	IN	NS	c0.nic.io.
io.	86400	IN	NS	ns-a3.io.
io.	86400	IN	NS	a0.nic.io.
io.	86400	IN	NS	ns-a2.io.
io.	86400	IN	NS	ns-a4.io.
io.	86400	IN	NS	b0.nic.io.
hostname.bind.	0	CH	TXT	"ns087b.app12.lax1.afilias-nst.info"
hostname.bind.	0	CH	NS	hostname.bind.
shl.io.	86400	IN	NS	ns2.d-zone.ca.
shl.io.	86400	IN	NS	ns1.p23.dynect.net.
shl.io.	86400	IN	NS	ns1.d-zone.ca.
shl.io.	86400	IN	NS	ns4.p23.dynect.net.
shl.io.	86400	IN	NS	ns2.p23.dynect.net.
shl.io.	86400	IN	NS	ns3.p23.dynect.net.

; <<>> DiG 9.8.3-P1 <<>> +noall +norecur +cmd +answer +authority -4 +notcp @c0.nic.io. io. IN SOA hostname.bind. CH TXT shl.io. IN NS
; (1 server found)
;; global options: +cmd
io.	86400	IN	SOA	a0.nic.io. noc.afilias-nst.info. 1497256290 10800 3600 2764800 900
io.	86400	IN	NS	ns-a1.io.
io.	86400	IN	NS	c0.nic.io.
io.	86400	IN	NS	ns-a4.io.
io.	86400	IN	NS	ns-a2.io.
io.	86400	IN	NS	ns-a3.io.
io.	86400	IN	NS	b0.nic.io.
io.	86400	IN	NS	a0.nic.io.
hostname.bind.	0	CH	TXT	"ns087b.app25.iad1.afilias-nst.info"
hostname.bind.	0	CH	NS	hostname.bind.
shl.io.	86400	IN	NS	ns4.p23.dynect.net.
shl.io.	86400	IN	NS	ns2.d-zone.ca.
shl.io.	86400	IN	NS	ns2.p23.dynect.net.
shl.io.	86400	IN	NS	ns1.d-zone.ca.
shl.io.	86400	IN	NS	ns1.p23.dynect.net.
shl.io.	86400	IN	NS	ns3.p23.dynect.net.

====== Community DNS ======

; <<>> DiG 9.8.3-P1 <<>> +noall +norecur +cmd +answer +authority -4 +notcp @ns-a1.io. io. IN SOA hostname.bind. CH TXT shl.io. IN NS
; (1 server found)
;; global options: +cmd
io.	86400	IN	SOA	a0.nic.io. noc.afilias-nst.info. 1497256290 10800 3600 2764800 900
hostname.bind.	0	CH	TXT	"NORM 002590-E2B186"
shl.io.	86400	IN	NS	ns1.d-zone.ca.
shl.io.	86400	IN	NS	ns2.d-zone.ca.
shl.io.	86400	IN	NS	ns1.p23.dynect.net.
shl.io.	86400	IN	NS	ns2.p23.dynect.net.
shl.io.	86400	IN	NS	ns3.p23.dynect.net.
shl.io.	86400	IN	NS	ns4.p23.dynect.net.

; <<>> DiG 9.8.3-P1 <<>> +noall +norecur +cmd +answer +authority -4 +notcp @ns-a2.io. io. IN SOA hostname.bind. CH TXT shl.io. IN NS
; (1 server found)
;; global options: +cmd
io.	86400	IN	SOA	a0.nic.io. noc.afilias-nst.info. 1497256201 10800 3600 2764800 900
hostname.bind.	0	CH	TXT	"COMM 002590-E2B186"
io.	900	IN	SOA	a0.nic.io. noc.afilias-nst.info. 1497256201 10800 3600 2764800 900

; <<>> DiG 9.8.3-P1 <<>> +noall +norecur +cmd +answer +authority -4 +notcp @ns-a3.io. io. IN SOA hostname.bind. CH TXT shl.io. IN NS
; (1 server found)
;; global options: +cmd
io.	86400	IN	SOA	a0.nic.io. noc.afilias-nst.info. 1497256290 10800 3600 2764800 900
hostname.bind.	0	CH	TXT	"NORM 001B24-2DB609"
shl.io.	86400	IN	NS	ns1.d-zone.ca.
shl.io.	86400	IN	NS	ns2.d-zone.ca.
shl.io.	86400	IN	NS	ns1.p23.dynect.net.
shl.io.	86400	IN	NS	ns2.p23.dynect.net.
shl.io.	86400	IN	NS	ns3.p23.dynect.net.
shl.io.	86400	IN	NS	ns4.p23.dynect.net.

; <<>> DiG 9.8.3-P1 <<>> +noall +norecur +cmd +answer +authority -4 +notcp @ns-a4.io. io. IN SOA hostname.bind. CH TXT shl.io. IN NS
; (1 server found)
;; global options: +cmd
io.	86400	IN	SOA	a0.nic.io. noc.afilias-nst.info. 1497256201 10800 3600 2764800 900
hostname.bind.	0	CH	TXT	"COMM 001B24-2DB609"
io.	900	IN	SOA	a0.nic.io. noc.afilias-nst.info. 1497256201 10800 3600 2764800 900