This is largely because with every day of storage, vitamin C levels decrease by about 2%. Nevertheless, this does not necessarily mean that re-constituted juice is better, as the rate of vitamin C degradation may be greater in re-constituted juice. Therefore, the bottom line is, don’t leave juice in your fridge for too long, its benefits will decrease.
—I learned about this fact from: Terry LA, ed. Health-promoting properties of fruits and vegetables. CABI: Cambridge, 2011.