The reason that sail measurement methods and computer design softwares differ in sail measurement reading is because the software is using advanced math (derivation, etc) and getting a very accurate sail area figure. It is also taking into account the entire surface area that is formed by the 3D shape that you don't get when you lay a sail flat on the ground. Physical sail measurement methods generally use a series of defined triangles to approximate the sail area as well. Also, because sails aren't straight line triangles, but the prescribed measurements are, the result of a physical measurement is only an approximation of sail area - but not the actual sail area. The accuracy of a sail measurement system is correlated to the number of measurement points and since it's impractical to have (for instance) 3,000 measurement points, most systems make a compromise in the number of measured points vs. the accuracy of the result. However, if there isn't enough accuracy with the measurement system, the door opens to sail designs that work around the system and stuff sail area into the sail in ways that the measurement system cannot measure. It's all a balance in practicality, accuracy, and keeping things in check.