Symptom: You used mass floors to divide a mass. In a schedule, tag, or instance properties, you notice that the exterior surface area for the mass floors is not as large as you expect it to be.
Issue: The mass is joined to another mass. The area of the inner wall shared by the masses is deducted from the exterior surface area for each mass floor. It is also deducted from the gross surface area for the mass. See Overlapping Mass Faces.