reverse lookups with dig for internal domains

[Sai balasubramanyam]

Hi all 

I have been scratching my head for the past two - three days to come
to terms with  an inexplicable (atleast it seems so to to me ) 
behaviour of dig. Let me explain it ..

We have an  internal domain in the private ip space , which cannot be
looked up from external world. When I do a dig -x <ip> from our
internal name server

 say dig -x 10.1.1.1 

gives the host name right 

but the authority section is from the root zone 
----------- 
;; AUTHORITY SECTION:
10.in-addr.arpa.        9h6m59s IN NS   BLACKHOLE-1.IANA.ORG.
10.in-addr.arpa.        9h6m59s IN NS   BLACKHOLE-2.IANA.ORG 
 
------------

When I follow that up with a qury like 

 dig ns 1.1.10.in-addr.arpa 

I get the name servers right ( that of our internal domain ) 

and now when I  try to reverse lookup any ip in the internal domain
the authority section of the answer is coming out absolutely right 
ever after .

thoughts/comments ? 
Sai.