If your router has a real routable address the ISP may be up to no good and running your web traffic though a proxy server.
Try going to http://lookingglass.level3.net/
Choose the traceroute option and then put in the 2 different address you think you have and see what happens.
Key here will be to try the ping option from this screen and put in the ip your router thinks it has. This will show if that ip is visible to the internet.
It will depend on your router if you can use the dyndns options that let you specify the ip you want to map. Some routers are dependent on the dyndns site to discover this and if there is something like a proxy in the path your web traffic will take a different path than other traffic.
Always a challenge to figure out what tricks a ISP is up to.