That is because we truly only have one IP address.
The router uses NAT to separate your lan from the internet. It cuts down on the amount of ip addresses that you need to have. This is good for a couple of reasons... it makes our LAN private and it also cuts down on the number of ip addresses that are being used. It will also hide your identity on the internet... but only to your network, not to our machine as if you had a public ip address.
So... our LAN ip addresses are different, but our WAN ip address is the same.
The only way to change that would to either get a second ip address from your ISP which will cost more $$... or you can use a proxy. Proxies will hide your true identity but it is also possible for anyone running the Proxy to see what you are doing and possibly grab usernames and passwords if you enter them.
It is OK for you both to have the same IP.
http://searchwebservices.techtarget.com ... 07,00.html
^^^heres some more about NAT
http://www.geektools.com/rfc/rfc1631.txt <--and some more nitty gritty about it
just learned this today in my cisco networking class, pretty intresting =]