25% being none of the above doesn't mean 25% are atheist/agnotic.
The world fact book 2014 estimate has the numbers as: Protestant 46.5%, Roman Catholic 20.8%, Mormon 1.6%, Jehovah's Witness 0.8%, other Christian 0.9%, Jewish 1.9%, Muslim 0.9%, Buddhist 0.7%, Hindu 0.7%, other 1.8%, unaffiliated 22.8%, don't know/refused 0.6% (2014 est.)
I personally know a few people (not Americans), who believe in God but do not identify themselves to any religion, or spiritual movement. So out of those options they would chose "unaffiliated" not "other". Athiests / Agnostics might make up a majority of the "unaffiliated" group, but they don;t make up the totality of that group.
“The fundamental cause of the trouble is that in the modern world the stupid are cocksure while the intelligent are full of doubt.” - Bertrand Russell
"When the power of love overcomes the love of power, the world will know peace."
Jimi Hendrix
