Why do I have different IP addresses on different websites?

Chances are that what you are seeing as this "wrong" IP address is a transparent proxy between you and your Internet service providers connection to the internet.

This is basically what paxdiablo was trying to get across, with NAT you don't necessarily get local addresses assigned and it is actually possible that you have a fully accessible "normal" IP address (so you can run servers and voip clients that require incoming connections) but your actual data connection is through a transparent proxy that catches and caches a good number of websites so that they can do things like caching common websites and reduce overall bandwidth used for those sites.

What this means is that depending on how the site resolves your IP or whether the transparent proxy kicked in your address may appear differently to the site that is trying to trace you.

I have found this a couple of times with my isps in the past. If you try to do a 'whois' lookup on this odd ip address then you may well find that it belongs to either your isp or their upstream provider.

An alternative is a 'hostname' lookup which may resolve to something useful like proxy.somelocation.myisp.com as it has done for me in the past when I have encountered this seemingly odd situation.

Welcome to the wonderful world of NATting, firewalls, routers and other associated network infrastructure.

Your local IP address is rarely what the outside world sees you as simply because of NAT (network address translation).

Your ISP or local router will generally do this translation for you in such a way that many different people can share the same IP address (local ones like 10.x.x.x or 192.168.x.x, for example, and from memory so don't take the actual values as gospel). These local addresses are ... well ... local, so they don't get "published" to the real Internet.

That way, we didn't run out of IPv4 addresses back in 1986 :-)

ip138.com, www.ip.cn appear to be Chinese, the other sites are not.

If you are also in China, then perhaps you are being proxied to sites outside of China to avoid government firewalls (or alike)?