You're comparing two surveys. What are they? Were they asking the same question?
I found this, which is probably your source for the 1%:
http://pewresearch.org/pubs/1181/religious-identification-of-those-who-do-not-believe-in-god
5% say they do not believe in God. Of those, 24% identify themselves as atheists.
Here's another survey of the religious composition of America:
http://religions.pewforum.org/reports
16.1% unaffiliated, 1.6% atheists. About the same fraction.
So where did your 15% come from? I'm guessing you used this "unaffiliated" number from one survey and compared it with the "atheist" number from another.