The blog post linked below points out the percentages of total realtor listings on Zillow.  The Z Group didn’t take too kindly to such exposure, and they issued a cease-and-desist order, so I’m not sure how long this link will be working:

https://www.buildzoom.com/blog/zillow-realtor-listing-war

Zillow and the San Diego MLS do not have an agreement to automatically upload the realtor listings, so those seen on Zillow are manually-uploaded, or by private agreement.  The blog post shows that Zillow has about 90% of the San Diego realtor listings, but the same chart has four cities that show over 100% of the realtor listings.

How can Zillow have more than 100% of the listings shown on realtor.com?

The authors suggest that it could be due to quality issues, but let’s face it, it more likely due to sandbagging – realtors putting listings on Zillow to find their own buyers to double-end the deal, but not sharing them on the MLS.