Here is your possible answers, though it is my opinion the first is the correct one in this specific case.
Sellers agent has inlcuded an estimate, or perhaps actual measured, square footage of the finished basement area into the stated square footage in the listing.
The tax records (public records) show the finished living space as they were all built originally - without the basement area finished. obviously anyone can clearly see if one home is larger or smaller, but 25% than its identical neighbors. this one is not.
the other possibility would be if a home had an addition put on, or sometimes an attic finished off. In your case, no attic area to finish, and no addition is practically possible or generally desireable, in a middle-row home.
Feel free to contact me for more information, if you are not working with an agent, and want to find out more about homes nearby, or the buying process. Good luck under any circumstances.
Long & Foster Real Estate