dstiles - 7:27 pm on Jul 13, 2013 (gmt 0)
Not really familiar with managing DNS servers but guesses:
Caching from previous access
hosts file on the querying computer has an entry for the subdomain
subdomain not in DNS list - a subdomain has to be separately specified for an IP (in my experience)
if the subdomain is listed in another (eg external/public) DNS server then routing becomes important - the public server may take precedence.
Warning: be very careful about potential access by baddies - some DNS setups are very easy to crack and poison or subvert. But you already knew that! :)