Ok, they don't actually know. They just know how many people, in the 1990 census, had my first name (994,396) and how many had my last name (152,984) and they assume (incorrectly) that names assort freely. But it's fun anyway!
I know there's at least one other person with my name in my metropolitan area - search for my Amazon wishlist, which you should do anyway, because you never know when you might want to buy me a present, like for example for my birthday, which is in a few weeks. You'll see that there is SOMEONE ELSE. A Doppelganger, perhaps. A fiend who bears my name, but with the middle initial "G" and a much stronger interest in sci-fi.