AWS instance private IP changing after stop/restart (did not happen before)

Yes, I read an article by Eric Hammond here where he mentions that the private IP would also change when restarting. A few months ago, when I first got an AWS cluster up for hadoop, I used the internal IP to configure /etc/hosts and the internal IP wouldn't change (even when the instance is stopped, i can see the internal IP).

To replicate this cluster as part of our corporate account, I created a few AMIs and used those to launch the instances. Now, the IPs are changing each time the machine is restarted.

On checking the machines that did not have the IP change, there doesn't seem to be anything special about them. They are the same simple EBS backed instances with volumes. Hmm, so what's the difference between them?