about the additional section

[风河]

Hello,

I found that some queries have got the response which has additional
section, but some haven't.
For example, this query with www.google.com got the answer with
additional section set:

$ dig www.google.com

; <<>> DiG 9.6.1-P2 <<>> www.google.com
;; global options: +cmd
;; Got answer:
;; ->>HEADER<<- opcode: QUERY, status: NOERROR, id: 57737
;; flags: qr rd ra; QUERY: 1, ANSWER: 7, AUTHORITY: 4, ADDITIONAL: 1

;; QUESTION SECTION:
;www.google.com.                        IN      A

;; ANSWER SECTION:
www.google.com.         604672  IN      CNAME   www.l.google.com.
www.l.google.com.       172     IN      A       74.125.71.104
www.l.google.com.       172     IN      A       74.125.71.105
www.l.google.com.       172     IN      A       74.125.71.106
www.l.google.com.       172     IN      A       74.125.71.147
www.l.google.com.       172     IN      A       74.125.71.99
www.l.google.com.       172     IN      A       74.125.71.103

;; AUTHORITY SECTION:
google.com.             172672  IN      NS      ns4.google.com.
google.com.             172672  IN      NS      ns2.google.com.
google.com.             172672  IN      NS      ns3.google.com.
google.com.             172672  IN      NS      ns1.google.com.

;; ADDITIONAL SECTION:
ns1.google.com.         345563  IN      A       216.239.32.10

;; Query time: 0 msec
;; SERVER: 119.147.163.133#53(119.147.163.133)
;; WHEN: Thu Sep  1 10:50:45 2011
;; MSG SIZE  rcvd: 236



But this same query get the answer without additional section:
$ dig www.google.com

; <<>> DiG 9.6.1-P2 <<>> www.google.com
;; global options: +cmd
;; Got answer:
;; ->>HEADER<<- opcode: QUERY, status: NOERROR, id: 33219
;; flags: qr rd ra; QUERY: 1, ANSWER: 7, AUTHORITY: 4, ADDITIONAL: 0

;; QUESTION SECTION:
;www.google.com.                        IN      A

;; ANSWER SECTION:
www.google.com.         604800  IN      CNAME   www.l.google.com.
www.l.google.com.       300     IN      A       74.125.71.106
www.l.google.com.       300     IN      A       74.125.71.147
www.l.google.com.       300     IN      A       74.125.71.99
www.l.google.com.       300     IN      A       74.125.71.103
www.l.google.com.       300     IN      A       74.125.71.104
www.l.google.com.       300     IN      A       74.125.71.105

;; AUTHORITY SECTION:
google.com.             172800  IN      NS      ns4.google.com.
google.com.             172800  IN      NS      ns3.google.com.
google.com.             172800  IN      NS      ns1.google.com.
google.com.             172800  IN      NS      ns2.google.com.

;; Query time: 399 msec
;; SERVER: 119.147.163.133#53(119.147.163.133)
;; WHEN: Thu Sep  1 10:48:36 2011
;; MSG SIZE  rcvd: 220


My question is under what condition the nameserver answers the query
with additional section set?

Thank you.