You can actually claim a house with Zillow and update the stats. I was getting a HELOC earlier in the year and found out that they weigh heavily on the Zillow zestimate, so I claimed my house and updated the info that they use to compute value. You can change bedrooms, bathrooms, livable square footage, date of most recent remodel, etc. I believe @Anthony Dooley is correct for the homes that do not get claimed and updated, they will use the county tax assessor data as default values until someone claims and updates the data.
I actually exaggerated some stats on my property in order to get a higher value. If the bank is too lazy to do an actual appraisal, then I will take advantage and make my asset look even better (on paper). See the attached pictures - one shows the screen where you will find the "Edit Facts" button and the other shows the screen where you actually edit the stats.