Were you connected to the internet and did you buy the computer (as opposed to getting it as a gift someone else purchased)? I had the same thing happen this weekend; I suspect that it is pulled from Apple's system, which has that info on the buyer.
in Setup Assistant on you first boot you have to enter a telephone number plus other info such as email address and home address otherwise it wont let your go to the next step.
do you have a .mac account? It could have takken it from that
odd, I don't remember it asking me about telephone# and home address
I think it ask for full name and email address only. on Full name I just put my first nick name only.
After setup assistant was done. It show in address book my full name, home address and phone number.