True values, poor formulas

While many consider a valuable service, the methodology of applying a general formula to specific properties allows for completely false values.  If you say for example that values in town X decreased 1.5% this month and then just subtract 1.5% from every home in that town (zip) is inaccurate.  Example:  if you start a starting value on a property of $100,000 and then Devalue it by 1.5% a month, with no other positive factors, this will eventually give you a value of ZERO.  At what point does the zillow math account for the fact that it should never reach "zero"?  Realtors/lenders use zillow as an estimate,,,but come on, be real.  Waterfront homes loose 1.5% a month for the last 3 yrs?