That's pretty low based on the amount of people that actually are Christians.
You have to remember that it's based on the US, and the USA is entirely comprised of multiple ethnic backgrounds from the rest of the world.
Not everyone who populated the land came from a christian european country.
There are a fair amount of Eastern civilisations that have decendants in the US. They wouldn't necesarilly be Christians.
And I'd imagine this is including all sects of Christianity.
And then, not to mention the other main religions in the world;
Hinduism, Islam, Judaism, Sikkhism and Buddhism.
They'd make up a noticeable amount of the US as well.