these are two different things
1) DNS resolving -> lookup DNS name to obtain IP-address
2) configuring DNS servers on the client -> to specify what DNS servers to use
google does not know anything about your internal addresses
so if you use 8.8.8.8 (google-dns) as DNS server to query, only public ip-addresses are returned
if you use 10.100.20.10 (internal-dns) your internal address is returned
-> this is expected behavior
1) depending on the OS you may specify a primary and secundary,
where the primary is used first and the secundary if the primary is unavailable
2) if query is alternating between DNS, then you get alternating ip-address returned
3) if the query is sent to both DNS you get random response depending on which response is received first
