I have observed that at most times, the difficulty for a problem in a problem set is different from its difficulty in the scoring distribution during the announcement of the contest. Does anybody know how is it calculated and why it takes so much time to get it updated, sometimes weeks?

P.S. I think that scoring distribution is just for the relative difficulty of the questions that will appear in the contest and problem set difficulty is just a different concept but still, I want to know, how are they calculated and what takes them so long to get updated. Thanks!