Why is my router assigning the wrong address via DHCP

Are you sure there isn't a second DHCP server on the network?

I had a similar issue. I had plugged in my Vonage adapter to my router using the "PC" connection instead of the "WAN" connection. The "PC" connection was acting as a DHCP server on the network and assigning IP requests to devices, even to devices that were connecting via the Router's wifi connection! How crazy is that?