Looking at the data helps. 12 percent of those 21 believe in an impersonal force, which can basically mean anything, including kharma, and 3 percent say they don't know. That both still qualifies as atheist. The remaining 6% are weird though.
They also didn't give the actual question, which might have ranged from:"Do you believe in a God?" to "Do you consider the existence of a God possible?" Might account for the 6%.

Edit: Mormons are the only ones with a perfect score, although even they have 2% "Don't know/Other". And "Don't know" is weak atheism.