iftop is a low level tool which looks at the IP addresses which make the connection to the webserver. If you use a proxy server, this will always be the IP address of that server. Apache can use an extra header in the HTTP protocol called X-Forwarded-For. If the proxy server sets this header--and apparently your pound server does--then Apache can determine the original server address and use it in its logs.

To catch the original IP addresses with iftop, you'll probably have to move it to the pound server.