In the example with the 3 side ponds, the lock would be emptied in 4 stages. So at the start of a cycle of 4 'openings', the top quarter of water will go into the top pond, next cycle it will effectively go from the top to the middle, then middle to bottom, then bottom to out of the lock. You could say that this 'bit' of water is all that is lost in a cycle, so only 25% of the lock is lost per cycle. This means 75% must be saved for re-use. As everything would have to be amazingly efficient to achieve exactly 75% saving, they probably say they only re-use/save 60% per cycle.