Not 100% sure how to explain it. If you pour water directly on the ground, it soaks in fairly quickly. If you pour it into a 5 gallon bucket filled with dirt (and the bottom cut out) it won't start soaking into the ground below until there the dirt cannot hold any more water. So a light rain or a short rain, may not every soak into the ground below and is only being used by the plants and via evaporation. A good mulch to prevent evaporation and the only water the plants do not get is when there is excess water. When planting in the ground, the moment the water enters the ground it begin to soak into the surrounding area also, so it takes much more water to get the same depth of water as the raised bed.