Simply because they poll everyone, and not the people who actually vote.
A poll “may” give insite into what people may do, but unless ever voter is polled and every voter is honest, you will never know.
Several of the polls polled “likely caucus goers”, but that model is based on who has gone to caucuses before.
The prior record for a Republican caucus in Iowa was around 122,000 votes. They had around 170,000 this time. So that blew up all the models.
Now, next time they’ll plan for a big turnout like this and if they get a more typical turnout, that will blow up the model again.