Why are the numbers for California larger than for the United States?
I think because when you set up your profile page, you have the option of listing either your specific state or the United States. So the United States numbers only show people who chose the U.S. instead of their individual state.
To get the true numbers for the U.S., I think you'd have to add up all the states plus the "U.S." people.
Maybe Jim Robinson can tell us if my theory is wrong :)