Meaning, Utah homeowners are not required to disclose their sale prices. And the Realtors are not required to share their information with Zillow either. So Zillow CAN be accurate in some circumstances - but their sampling of sold homes is so spotty that their values may be off. I've seen them WAY off.
Same with Tax Evaluations. I just discovered my Dad's home was valued at over $100,000 MORE than it is actually worth (they're usually less than market value). I even called the County and confirmed with the County Assessor who when looking at my Dad's file said, and I quote, "woops"