Comment 1 for bug 657270

After doing more research, I believe this is because Apache is not using the IPV6_V6ONLY flag when it binds to port 80. This allows the IPv6 socket to serve both IPv4 and IPv6 traffic. Since there is technically only one listening socket, this is why netstat only shows the IPv6 socket.

Historically we could use netstat to see exactly what is listening, but in the case of IPV6_V6ONLY, it's entirely possible that an IPv6 socket could handle IPv4 traffic and netstat would never show it.

Here is the README from the netbase package:

# When disabled, IPv6 sockets will also be able to send and receive IPv4
# traffic with addresses in the form ::ffff:192.0.2.1 and daemons listening
# on IPv6 sockets will also accept IPv4 connections.
#
# When IPV6_V6ONLY is enabled, daemons interested in both IPv4 and IPv6
# connections must open two listening sockets.
# This is the default behaviour of almost all modern operating systems.

IPV6_V6ONLY is controlled by the sysctl net.ipv6.bindv6only.