I have looked at Zillow many times and compared the houses on my street which were all built within a year of each other and of similar size with the exception of mine. My house is about 1000 sqft to 1500 sqft larger and is the second largest house on the street, Zillow's listing of the price per square foot is drastically different, about a $70 per square foot delta.
My house is $40 per square floor less than my next-door neighbor's house for near identical looking houses, my garage is larger as well as the number of bedrooms, etc.
I noticed that all of the houses were similar in size but with a delta in price and without Zillow knowing the detailed differences between them, Zillow is using some sort of Min-Max pricing with a mean value for houses on this one street, regardless of the size. I noticed that the one house that is larger than mine, is priced per square foot lower than any other house on the street, putting it at the max value for the street, not by much and twice the size.
Using the average per sq ft value of the other houses would put the one large house at nearly double the price of every house on the street, except mine. In the end, it means that if this person sold their house, the buyer would be getting a great deal, a much larger house with the price based on smaller homes on the street.