Why such a huge discrepancy you ask?
Well, that's because both sites use computers and mathematical equations along with public records to generate their estimates......computers can be right, and they can be wrong. Each site's mathematical algorithms may vary and are based on their own site's use of the stats.
Nothing can beat an actual live person viewing the property and selecting accurate comps that will help reflect the real market value of the home....
Hope this help ease your concern.