Because the height difference that matters is the difference between the water level in the lake and the turbines. If your pumping station inlet is 20m below t

Because the height difference that matters is the difference between the water level in the lake and the turbines. If your pumping station inlet is 20m below the turbines, then any energy used pumping water upto the height of the turbines is wasted, because it can't be recovered when the water comes back down.