I think that's the point.
Cox (and my understanding many other ISPs as well) CHARGE for static IPs, but give out dynamic IPs that basically never change because it is actually just easier for them to set the service up that way.
The few times the IP address changed was when they did an upgrade at their site, a very long outage, and I think a time when an installer screwed up a neighbors installation.
I cannot guarantee my address tonight will be what it is now, but the reality is, it will be.
I believe (stress no real evidence of this) that my home router tries to give out the same IP addresses to the same MAC addresses. I say this because after I reboot the router, my various machines will pop up at the same IP addresses they had before (and none of them are Apple products known to do this on purpose.)