I don't know why it's doing that. Maybe you're doing something wrong, maybe this MOD is. In order to figure it out, however, you'd need to provide me with FTP access.
That said, I consider IP Unmasker
/ Proxy Revealer to be the successor to this MOD. You might want to consider installing that one.
I should mention, however, that it works rather differently then this MOD does. IP Unmasker / Proxy Revealer can detect HTTP proxies and so-called CGI ones. It, however, is not able to distinguish between closed and open HTTP proxies as this MOD can. This MOD also cannot catch CGI proxies.
Finally, this MOD is more affected by server configurations then IP Unmasker / Proxy Revealer is. For example, if your web host is using NAT, this MOD will yield 100% false positives whereas IP Unmasker / Proxy Revealer will be unaffected.