anyone know why Zillow randomly changes Z-estimates, sometimes several times in a week?

I want to caution when using these sites, that there is no where that gives accurate property values. What makes up a property value is much more complicated than finding a fair price for a used car. Past sales of similar properties seams simple enough but was it flat or on a 40 degree slope. Cleared or full of trash, street noise, these and more can vary in a block or two distance. You need to contact a local appraiser or realtor to get the fair asking price. These computer programs are unable to give a reliable price estimate because condition and often time square footage are not available for the programs. A 3 bedroom 2 bath 1000 square foot fixer upper and a 3 bedroom 2 bath 3000 square foot in new condition have little effect on each others value.  There is a lot of fantastic useful information on these site and others like it. But property values are not among them.