Which Forwarder Does Bind Pick?

[Jiann-Ming Su]

In a simple fowarding only name server config:

options {

    forward first;
    forwarders {
        10.220.0.34;
        10.220.0.38;
    };
}

How does the named process determine when to use one forwarder or both forwarders?  I'm sniffing the traffic and on some queries, it goes for the first one.  On other queries, it goes for both.  Thanks for any clarification.