In any survey you get a certain persentage whose answers simply make no sense, such as people who don’t believe in God but think the Bible is inspired by God or some such nonsense. The problem is words can be construed in many ways. For example someone might say God inspired the Bible the way angels inspired Michaelangelo. Or people may simply be careless in how they answer, or have genuine convictions that are inconsistent. All of us are capable of holding beliefs that are inconsistent if pursued to their logical conclusions (my belief I should drive my car and my belief that global warming is a bad thing). Sometimes people are just irrational too.
Having said all that, don’t Buddhists believe in an afterlife without God as such. So it’s not an unknown position, if a little unusual to western conceptions.
Plus you've got people with non traditional conceptions of God, that might fall outside most notions of “God”: such as pantheists, animists, panpsychists and so on.
Plus you get singularitains these days, some of whom seem to espouse a kind of afterlife without God.
So many possibilities in this amazingly unfathomable universe of ours.