It could be reasons like the other agents have said, but if you want the exact details contact the listing agent and ask them to email you the listing! Hope this is helpful.
Also, it is important to note that the county appraisers do not actually go inside the home to measure. They measure around the outside of the home to get the square footage measurement, which isn't always the most accurate.
Finally, the home in question states the source for sqaure footage as "Homeowner." Well, in my experience, this is not a good source for a measurement. I would take the tax records over the homeowner's word unless they can show me evidence of the remodel/addition or a legit measurement from an appraisal.
The homeowners should be able to tell you why there is a discrepancy. Where did they get that number?