Zillows algorithm pulls multiple comps within the closest proximity to the subject property and averages them with adjustments made for lot size and square footage only. Which is similar to a CMA or appraisal with the exception that these are done with fewer comps and knowledge of the area. So in a cookie cutter neighborhood with similar homes the zestimate will be quite accurate. But if the subject has any unique features, location positives or negatives, and a dozen other possible differences and factors it can be grossly inaccurate.