a domain's NS record point to itself

As I understand that if I have domain A.com, it must be managed by another domain like ns1.B.com, ns2.B.com

$ dig ns alibaba.com

; <<>> DiG 9.9.2 <<>> ns alibaba.com
;; global options: +cmd
;; Got answer:
;; ->>HEADER<<- opcode: QUERY, status: NOERROR, id: 45070
;; flags: qr rd ra; QUERY: 1, ANSWER: 4, AUTHORITY: 0, ADDITIONAL: 0

;; QUESTION SECTION:
;alibaba.com.                   IN      NS

;; ANSWER SECTION:
alibaba.com.            76962   IN      NS      nshz.alibabaonline.com.
alibaba.com.            76962   IN      NS      nsp.alibabaonline.com.
alibaba.com.            76962   IN      NS      ns8.alibabaonline.com.
alibaba.com.            76962   IN      NS      nsp2.alibabaonline.com.

;; Query time: 6 msec
;; SERVER: 192.168.1.28#53(192.168.1.28)
;; WHEN: Wed Dec  3 18:56:43 2014
;; MSG SIZE  rcvd: 117

but what I don't understand is:

 $ dig ns aliedge.com

; <<>> DiG 9.9.2 <<>> ns aliedge.com
;; global options: +cmd
;; Got answer:
;; ->>HEADER<<- opcode: QUERY, status: NOERROR, id: 48684
;; flags: qr rd ra; QUERY: 1, ANSWER: 4, AUTHORITY: 0, ADDITIONAL: 0

;; QUESTION SECTION:
;aliedge.com.                   IN      NS

;; ANSWER SECTION:
aliedge.com.            140046  IN      NS      ns4.aliedge.com.
aliedge.com.            140046  IN      NS      ns2.aliedge.com.
aliedge.com.            140046  IN      NS      ns3.aliedge.com.
aliedge.com.            140046  IN      NS      ns1.aliedge.com.

;; Query time: 6 msec
;; SERVER: 192.168.1.28#53(192.168.1.28)
;; WHEN: Wed Dec  3 18:58:32 2014
;; MSG SIZE  rcvd: 101

aliedge.com is authorized nameserver to it self, how can this happend?