What's My IP Tool shows your public IP address - the IP address seen by other devices and servers on the Internet.
Is see a different IP address on my computer?
Most networks do some form of address translation called NAT to enable support for multiple devices given the limited supply of public IPv4 addresses. Your network router will be responsible for forwarding traffic between your external IP address and your internal IP address.
What is the other IP show found in header?
Some routers and proxy servers add a special header HTTP_X_FORWARDED_FOR to the request to indicate which device or computer is actually making the request. This is not always available.