Earlier quoted context omitted.
>I wonder what would her argument be if Zillow simply reported the City/County tax assessors estimates which are usually even lower than Zillow's. County assessors for tax purposes are explicitly not trying to determine market price. Zillow should ignore it.
I'm not sure what you mean. In Illinois (the state from the article) the assessor is calculating the market value: Most real property in Illinois must be assessed based on its value on the open market, or its “market value.” This value is the amount at which a property would sell in a competitive and open market... [1] [1] http://tax.illinois.gov/Publications/LocalGovernment/PTAX100...
See http://www.investopedia.com/articles/tax/09/calculate-proper... for different methods used. As an example, some places merely look at the cost of replacing the house if destroyed.