about the additional section

[风河]

2011/9/1 Daniel McDonald <dan.mcdonald at austinenergy.com>:
> On 8/31/11 10:13 PM, "风河" <shorttag at gmail.com> wrote:
>
>> 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
>> ;; MSG SIZE  rcvd: 236
>> $ dig www.google.com
>>
>> ;; MSG SIZE  rcvd: 220
>
>> My question is under what condition the nameserver answers the query
>> with additional section set?
>
> When there is sufficient space for the additional information in the packet,
> it is sent.
>

But as you see, both of my queries have the size of 200+, less than 512 bytes.
So why they didn't include the full additional sections?

Thanks.