Agnosticism is the separate category in that questioneer. Pew is weird about it, they just list every major religion and sect, then "other" then "agnostic", "atheist", and "nothing", and you need to chose one, which might be the source of confusion, and I can't see any good explanation on why do they do it like that. LIke I said, bullshit number. "Don't believe in any gods, don't follow any religion, not an agnostic" is an atheist, by definition. Separating it into "atheist" and "atheist but different word" can only serve one purpose, to dilute the numbers so christians don't feel threatened by all the evil heathens.