Sq Footage Disclosure?

Streeteasy lists square footage sometimes on their sale pages (an example which is below). Where does this figure come from? It isn't from the brokerage listing, which is often extremely different from the figure on the sale pages (in part due to the well-known issue of real estate brokers exaggerating sq footage in their listings). In the example below, the listing say 2800 sq ft, and the sale says 400 sq ft. Clearly in this case the broker sq ft is more accurate, but the opposite is often the case. Does anyone know where the sale sq ft figure comes from? I have looked in ACRIS but have been unable to find it. http://streeteasy.com/nyc/closing/1804183