alt Hacker NewsBecause they can correct against the election results.
If you're trying to do a scientific poll that is not about an election, it can become exponentially harder unless you can find independent ways to obtain data that helps you adjust (like home ownership, which is reported to the government).
Retroactively but thats not why anyone looks at polls. By that point you’d throw out the poll and deal with the real result. And really theres no shot whatever corrective factor remains the same for every election and poll relationship. There are too few elections over too long time over a constantly shifting electorate to generate enough power to appropriately qualify this divergence.