In a constant-cost industry in which firms have U-shaped average cost curves, the long-run market supply curve is a horizontal line. This market supply curve is not the horizontal sum of individual firms' long-run supply curves. In this respect, the long-run market supply curve differs from the short-run market supply curve, which, in a constant-cost industry, will equal the horizontal sum of individual firms' short-run supply curves. Why does the derivation of the long-run market supply curve differ from the derivation of the short-run market supply curve?