Why non-authoritive?

[Patai sangbutsarakum]

Hi DNS's folks

        I 'm new bies in run DNS. why some time when I used nslookup --> set type=ns --> domain.co.th
I found this answer.

Non-authoritative answer:
domain.co.th   nameserver = ns2.domain.co.th
domain.co.th   nameserver = ns.domain.co.th

Authoritative answers can be found from:
domain.co.th  internet address = 203.xxx.yyy.zzz
domain.co.th   internet address = 203.xxx.yyy.zzz

But if I change quried server to be other and do the same thing again for same place. I just found.

Authoritative answers can be found from:
domain.co.th   nameserver = ns2.domain.co.th
domain.co.th   nameserver = ns.domain.co.th
domain.co.th  internet address = 203.xxx.yyy.zzz
domain.co.th   internet address = 203.xxx.yyy.zzz

why someone suggest me about this appear? is it some incorrect in my DNS.??

Thanks in advance.
Patrick.