I'm quickly looking this over and was wondering if this has to do with DHCP leases not expiring yet?
Example: A computer gets a DHCP address that expires in 12 hours.
You set a static IP address on the router.
You go to double check the computer's address and it's not the static address.
After 12 hours, the computer gets a new address because the DHCP lease expires and sees the static address now which it sets and stays the way until the end of time
(or the router settings get changed again)