strange dig response querying my subdomain zone

[fddi]

Hello.

if I query my domain zone with dig I have correct values as response.

dig mydomain.com SOA


now if I query one of my subdomains

dig subodmain.mydomain.com SOA

in the response I have a
ANSWER SECTION and AUTHORITY SECTION.

but I do not have an ADDICTIONAL SECTION and I did not expect it to happen.

What this should be due to ?
what kind of error in the configuration ?
I am sure the delegation and glue records are OK.

thank you

Rick