What does "localhost" mean in named.conf?

Configuring BIND 9, run across an interesting issue. What does the following excerpt from named.conf mean:

options {
        listen-on port 53 { localhost; };
        ...
};

I thought this tells BIND to listen only on loopback adress (127.0.0.1). After all, this what "localhost" usually resolves to. To my great surprise, I've found that BIND is listening on all network interfaces. As it turns out, in the context of BIND configuration, localhost "Matches the IPv4 and IPv6 addresses of all network interfaces on the system." Go figure! The correct configuration is as follows:

options {
        listen-on port 53 { 127.0.0.1; };
        ...
};