Why? Because superman doesnt unleash his full power but wait until you see his power in injustice. But thats not all maybe you didn't see the supreme or ultra mode of superman. And maybe you dont know the Infinite Punch of superman that can destroy a planet. So if superman and the sentry or the void faught with their full power and no holding back Superman would definitely win. And btw in his supreme mode He is so freakishly fast.