The appraisal districts are often very wrong in the square footage they have listed for a given property.
Many listings will note that the home is bigger than what is listed in MLS and the tax records but will not be more specific than that. The reason they continue to use the information from the appraisal district is a CYA type reason.
Sometimes, a property will be listed and the source of the square footage will be an appraiser. It will look something like
SqFt: 1,520 / Appraiser or
SqFt: 1,520 / building plans or
SqFt: 1,520 / Other Documentation
Other reasons the appraisal districts are wrong could be screw-ups or typos, an addition to the house which was never properly documented, a post-construction completion of a "bonus room". Many times, the "bonus room" will be completed with the rest of the home, but the tax office just used the base model when it made it's initial assessment.
There are numerous ways or reasons a house may be listed incorrectly. Don't worry about it or fly off the handle, just get it measured to verify what it actually is.