I'm sure there is some change in flow with regard to distance away from the source, because as some water flows out the nearer outlets, the pressure will be decreasing gradually, the further away from the source you get. So, I'm sure there is some sort of pressure gradient under the filter plate. However, once you start putting gravel at different thicknesses, plants, decorations, driftwood, castles, etc etc on top of the plate, this throws off the flow dynamic in ways that would be impossible to predict. Its probably easier to do it the way its done and leave the rest to chance. That is, assuming that the flow gradient is even great enough to actualy cause issues, which I'm not sure it is.