Public records (tax records) are historically inaccurate so when a seller or listing agent has a more reliable source (prior appraisal, builder's floor plan, etc.), they will advertise the square footage of the source they deem most accurate and the two often conflict.
Arturo, sometimes the tax authority does not realize there have been changes in a property. One example could be an enclosed porch that is now heated and cooled, which adds square footage to the home. If you have questions about specific properties, I'd be happy to look into them and try to find the answer! firstname.lastname@example.org
I am not sure which home you are speaking of, but often times, there is a difference between square footage on tax records and a listing. The difference could be from several things - the listing used a square footage from a recent appraisal that varied slightly from the one on the tax rolls; the listing may include additional square footage from a room addition; the listing may include additional square footage from a garage conversion, etc.....
Many thing can cause variances in square footage from a historical record to a present value.