A good article with evidence here by Ryan discussing the accuracy of the zestimates, and how Zillow calculates their success rate based on their updated zestimates once a property sells.

His comment section is full of examples where Zillow adjusts their zestimate to the list price once a property comes on the market, which is a practice Spencer denied on twitter below when discussing their prize winner for best algorithm improvement:

Ryan’s comment section will make you think that it is still an on-going practice for zestimates:

http://sacramentoappraisalblog.com/2019/05/01/two-things-to-understand-about-zillows-accuracy-rate/