This nerd stuff intrigues me. How can an IP address change for one person?
IP addresses owned by a particular ISP can/do change (e.g., AOL, Comcast, etc, each own a "batch" of IP addresses that are assigned dynamically and can change whenever you log in); but when those IP addresses are from different ISP's, that's another story...
There are what's called Static IP addresses and Dynamic IP addresses.
A static IP is one that is assigned to the user, a
permanent address. The problem now is that with the internet boom that has taken place there are no longer enough static IP addresses to give out on a permanent basis.
Dynamic is an IP address that is a temprorary address. One that is assigned on a temporary basis.
Then there are just people in general who think that they are actually fooling others just because, as Don Geoff has said, their IP addresses are from different ISP's. That is totally another story. Both one worth telling and one worth hearing.