1. It is illegal to advertise a certain number of square feet if they do not actually exist.
2. We have a lot ot "Ohana" living situations, where lanais are enclosed, decks are built and rooms are added on.
These are not always done by getting permits, especially 20+ years ago. This a good reason for a home inspector to look over everything carefully. Even in homes where the square footage is the same, you may see bathrooms modified, walls constructed or removed and some suspect electrical connections.
When in doubt, verify everything including square footage. Ask lots of questions.