The other answers are on the money. For Tax records the square footage is taken mostly off the plans, when under construction the plans measurements represents the stud face. Then on the studs you add the sheathing and then the siding, that alone will add from 1-1/2 to 5 inches to the outside of the wall. so if an agent is figuring the square footage of a property by measuring outside to outside a property will be from 3 inches to 10 inches larger all around. Even though locally we use square footage, many areas do not you can build a 1,000 sq, ft. house for the same value as a 2,0000 square foot home, its all in the finish and amenities included. In plain english its not always the footage but the quality.
By the way, you are buying what you see and how the house fits you. Square footage is an insteresting exercise, but should not be the main focus of your search for a home.
While I can't speak specifically about the measurements of the home, I can share some general information on home measuring that hopefully will help.
When a licensed real estate agent markets a home for a seller that agent takes own responsibility for advertising correct square footage measurements (within a reasonable tolerance level)
The square footage is calculated based on outside dimensions - if the home was a small 1 story ranch with no bay windows etc, just a perfect rectangle, then the calculation would be length x width to yield the square footage
If the home is a 2 story and the entry way is open 2 stories you would only count that space once for the 1st floor - the larger the home and the more angles etc the more difficult the measuring. Due to more complex measuring you could send 3 people to measure a larger home and receive 3 different numbers - all should be close but rarely would they be exactly the same.
The county does not go inside the homes to gather specific information - a real estate agent listing a home should never rely on the county tax site for accurate square footage.
It isn't all that uncommon to find a home measuring out at larger square footage than reflected on the tax records - most homeowners will not voluntarily ask the county to increase the size of the homes square footage because that would trigger a higher property tax bill, while of course if the county is showing the home larger then the typical homeowner will file to have that corrected.