Part of the problem has to do with the leftist tendency to gang up on people if they don't say the politically correct thing. Faith is taboo since it is the one virtue, more than any other, that implies you believe in God. The rest can be done by anyone, including an atheist, and are therefore more PC. Polls often reflect PC and not the truth in people's heart, because many people try to avoid the consequences of PC.
It is like if someone said they liked Trump. This is taboo in many circles. To say that answer, one has to be willing to put up with people attacking you and not just your opinion. It is easier to say Bernie, rather than deal with this. The good Christian will turn the other cheek, but doesn't want to be slapped for being honest.
Bullies know this and can use the threat to make the polls work out in ways, that do not reflect reality. The goal is, if enough people give lip service, it will become the collective perception of truth. It is like tail wagging the dog due to relative perception. If you believe in relative truth and relative morality, the tail can wag the dog.