mmm, it's because the self test will tell you your external address, but this is not the one you should tell your friend.
I'll try to be detailed but clear.
You actually have 192* address, and outer 172* address, and then an internet address 116*.
This is while you go from inside to outside (the internet).
The 192* will work only in your apartment (or those who connect to your router).
The 172* will work for those who use the same cable net.
The 116* will work for no body, since you cannot configure the outer router to forward the port
So tell your friend your 172* address.
If you don't know such address, you can
1. read it in your router web administration
OR
2. open the command line (prompt) and use the command: tracert google.com