It's unclear to me how much visibility Zillow really had, since, AFAIK, they don't have access to MLSs in the areas where they buy and sell.
I don't think it's hyperbole to say Zillow has more data on residential real estate than any other company in the US.
My understanding is that Zillow relies on realtors to add their listings, so that means, maybe-it-happens, maybe-it-doesn't, entry errors, maybe-it's-updated, maybe-it's not...
Source: worked on MLS ingesting software.