When “CBS This Morning” co-host Norah O’Donnell asked the chief executive of Zillow recently about the accuracy of the website’s automated property value estimates — known as Zestimates — she touched on one of the most sensitive perception gaps in American real estate.

Zillow is the most popular online real estate information site, with 73 million unique visitors in December. Along with active listings of properties for sale, it also provides information on houses that are not on the market. You can enter the address or general location in a database of millions of homes and probably pull up key information — square footage, lot size, number of bedrooms and baths, photos, taxes — plus a Zestimate.

hoppers, sellers and buyers routinely quote Zestimates to realty agents — and to one another — as gauges of market value. If a house for sale has a Zestimate of $350,000, a buyer might challenge the sellers’ list price of $425,000. Or a seller might demand to know from potential listing brokers why they say a property should sell for just $595,000 when Zillow has it at $685,000...