I think you are overestimating the intelligence required to get a degree in this country, also lots of intelligent people have religious beliefs of some level.

And most people who don’t necessarily believe in god or practice any religion still respond to such questions with whatever religion they were born into because it’s not important enough to them to take a hard stance like calling themselves atheist, or maybe they choose to be agnostic so they might not pick the atheist option.

My point is lots of factors go into surveys like these, they don’t necessarily paint a super accurate picture, since any type of survey will have some external and internal biases and sampling issues baked into them