This is a perfect example of why any source of information that is based soley on tax records is unreliable as they can be anything from accurate to way off. That's why you need too visit properties. It's very frustrating for us professionals as well, especially when the square footage isn't listed at all.
And Jed is correct about sq footage not being an exact science. Some sources include stairwells and walls. For example, one client was buying a property that showed 1507 sq ft in the MLS remarks but only 1385 in the condo map. From my perspective to knowingly include non-usable space such as stairs and walls in the square footage number is ridiculous and a gross misrepresentation of the property, yet it is still done.
Foregoing aside, even if all the properties had accurate square footage using the same criteria, when viewing a number of properties you're going to come up with an average price. This is not really a useful number in San Francisco where there are massive differences in building age, condition, views, proximity to projects, traffic noise, proximtity to public transportation, legal issues, etc...
The best thing you can do is make sure you have a representative who is knowledgeable about all these issues.
Lance King/Managing Broker