Diferente IP del dispositivo Android (3G) y whatismyip.com?

Why do I get a different ip using:

Obtener la dirección IP del dispositivo Android cuando está conectado a una red móvil 3G

That I do by going to whatismyip.com and is there a way to get the "real ip".

The difference that I have seen between the two ips is that sending UDP packages to the whatismyip.com ip get through much faster than the ip provided by android.

I also saw in logcat that the ip address that whatismyip.com is being found by someother service:

I/pppd ( 3178): local IP address 107.52.xxx.xxx
I/pppd ( 3178): remote IP address 66.1.xxx.xxx

So there must be a way... or some other service is getting the ip from some web service