The algorithms that Zillow uses are very complex. They take many factors into account including square footage, number of bedrooms, bathrooms, recent sales, input from site users, etc. They may not have all the facts for the units or the information they have could be incorrect. If it's your unit, you can "Claim the Home" and edit the facts about the unit. That should help them get more accurate data and get the pricing more in line.
It looks like they are updating it. Is there an assessment for the work?
This home was built in 1950