You might be surprised about places other than SoCal and NY NY. Like growing up in Tennessee Alabama and Georgia, everyone seemed to have some affiliation. The level of engagement varied, but I never met someone that was an openly staunch atheist in real life. There is a deep stigma about such a thing in the South, - sadly. There, even extremists like Church of God are nominalized (screaming you're going to hell for an hour, exorcism/miracle drama nonsense, mobbing behaviors, religious masochism).

It is likely one would need to be second or third generation removed from and religious social support network in a family unit before a person would truly answer atheist on such a poll. I don't think many people grow to the point of self awareness to care to define an anti religious god certainty at this stage in human cultural evolution.