Excellent article about home values from the LATimes…
http://www.latimes.com/business/realestate/la-fi-harney-20150208-story.html

Zillow is simply off on its home values. At its best its 7% off. Commonly it 12% off. At its worst 40%

Why?  Home are  Un-Zillowable.

Zillow has no idea if your house is Updated, needs a new roof, has termites, backs up to the interstate or a golf course. Those are all Un-Zillowable factors that can dramatically effect value.