A clarification...when you run the IPCONFIG /ALL you should be doing this from the laptop that is obtaining a DHCP address.
It's possible that the Unix system is configured to pass out IP addresses.
On the whole DHCP versus static addresses issue, DHCP does a great job of taking the work out of assigning addresses. But you also have a problem in that any system that physically plugs into your network can grab an IP address, even if the system shouldn't be there.
Some companies that I've worked for have been very meticulous about IP address management, assigning or reserving IP addresses for only machines on our network. This way vendors or other visitors couldn't just plug into our network and expose us to whatever sort of virii/worms are on their system (or use our network to surf the web for porn, launch DDOS addtacks, etc). Of course, if the person plugging in is remotely tech savvy then they'll probably be clever enough to manually assign an IP address to their system inside the scope that you are using, and thereby still cause problems for you. But since most visitors/vendors to my company tend to be about as technically sharp as a turnip, it seems to work OK.