Because the IP address assigned to the box may not be the IP address it is visible to the outside world on?
The output of ifconfig on my mac is dramatically different from what http://jsonip.com/ is showing me, and for good reason.
In addition to standard NATing, if he were on an EC2 instance, the IP addresses are routed to by Elastic IPs, and aren't necessarily visible from the box itself either.
Simply put, there are a variety of reasons you wouldn't necessarily want to rely upon the system's local IP address as canon for what others might see it as.
