It doesn't. People are not robots, and therefore perform differently. They react to criticism differently, they have different abilities, they have different ambitions, they have different motivations. Treating everyone the same yields the lowest of those qualities as there is no incentive to do better than minimum. If you don't believe me, look at government and union jobs, where everyone is treated the same and many people do the minimum to get by. People with ambition and abilities go where the differences are recognized.