if you are on a network, machines are constantly comunicating with each other always trying to find out whose boss, holding elections and just making sure no one has left or is new. although their is quite a bit of echo requests going on there is more than one type of ICMP packet.

if you can install a packet sniffer or network monitor on your machine it would be a very enlightening experiance to view all the traffic thats always there behind the scenes.

if you need to know more about the ip address discrepancy than has already be stated do a search for NAT or "network address translation"