Symptom: In a schedule, tag, or instance properties, you notice that the gross surface area for a mass 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 gross surface area for each mass. If you create mass floors, this inner wall area is also deducted from the exterior surface area for each mass floor. See Overlapping Mass Faces.
Solution: None required.