"ORPort [::]:9001" sometimes listens to both IPv4 and IPv6, sometimes to IPv4 only.

On some operating systems, an AF_INET6 socket listening on :: can also receive IPv4 connections, but whether it does by default is OS-dependent. It depends whether the IPV6_V6ONLY socket option is set.

You may want to set this socket option on all sockets resulting from an IPv6 Bridge line, for uniformity and to avoid accidentally listening on IPv4 when you don't intend to. Linux by default has this option set; Windows and FreeBSD apparently do not.

An observable effect of this phenomenon is that this configuration fails on Linux:

ORPort 9001
ORPort [::]:9001

The problem is that "ORPort 9001" binds to IPv4, and "ORPort [::]:9001" tries to bind to both IPv4 and IPv6.

Dec 22 05:58:48.104 [notice] Opening Socks listener on 127.0.0.1:9050
Dec 22 05:58:48.104 [notice] Opening OR listener on 0.0.0.0:9001
Dec 22 05:58:48.104 [notice] Opening OR listener on :::9001
Dec 22 05:58:48.104 [warn] Could not bind to :::9001: Address already in use. Is Tor already running?
Dec 22 05:58:48.104 [notice] Closing partially-constructed listener Socks listener on 127.0.0.1:9050
Dec 22 05:58:48.104 [notice] Closing partially-constructed listener OR listener on 0.0.0.0:9001
Dec 22 05:58:48.104 [warn] Failed to parse/validate config: Failed to bind one of the listener ports.
Dec 22 05:58:48.104 [err] Reading config failed--see warnings above.

http://tools.ietf.org/html/rfc3493#section-5.3 describes the IPV6_V6ONLY option.