nslookup can't but browser can !

[Quadri, Jay]

I disagree here is why:
Forward first causes the server to check the local cache for the answer and
if not found, then forward the query. This is the default setting.
Forward only the server will only forward the queries.

You didn't explain why 'forward only' does not forward to other Internal
nameserver. 
and why 'forward  first' does not forward to the Internet nameserver.

It's a mystery to me.  You might want to read the question again.