Ann: Mustang Discovers Diamonds in Mozambique, page-38

I understand your point that the higher grades near the surface would settle closer to the source. However, we are not talking about recent history. These processes have been occurring over millions of years. There are lots of variables in the past that can alter the gravel deposition. Extreme flooding events could have produced currents in the river strong enough to move the gravels a long distance. This would also cause only the heavier gravels (and diamonds) to become entrapped on the slower velocity points of the ancient river systems.