I don't know the subject specifically, but I would assume it has something to do with outside factors such as temperature, age, pressure, etc that aren't accounted for in the calculated fracture strength but exist in the real world. Most calculations, like calculated mass vs actual mass in chemistry formulas, differ because the calculated mass is produced in an ideal environment, where nothing wrong is happening, and no/a set amount of energy is being lost to the outside world. This becomes near impossible to reproduce in the outside world, so the results slightly differ.
