I have a DSL connection with dynamic IP.
My problem is that it seems that different pages that show my external
IP, display different results.
For example:
showip.net shows X.Y.Z.84
showmyip.com.ar shows X.Y.Z.86
mediacollege.com/internet/utilities/show-ip.shtml shows X.Y.Z.3
whatismyip.com shows X.Y.Z.116
showmyip.com shows X.Y.Z.3 (this time is the same
result as one of the pages above)
the No-IP DUC shows X.Y.Z.84

Of course the X Y Z don't change.
So it seems like for different pages or requests my ISP assigns me
different IPs or something, I don't really understand it.

Initially I wanted to make my home computer a small web server (nginx
listening on port 6789) that could be accessible from the
internet and since my IP is dynamic I tried DYNDNS (with no result)
and now No-IP (with no result, too).
I added the correct entries for port forwarding in Advanced -> NAT ->
Virtual Server in my ZTE ZXDSL 831II modem/router (fireware version
ZXDSL 831IIV7.5.0a_Z29_AL1) but still couldn't access my web server
neither from the DynDNS host or the No-IP host, so I slowly am giving
up to the idea of running a web server in my home pc.

But I still am curious to learn more about networks and to understand
why the above problem (the different WAN IPs problem) happens!

So why does that happen?
Is it some kind of technique used by ISPs to distribute requests or