Ann: Mustang Discovers Diamonds in Mozambique, page-40

I think you are missing the point.
Try it like this very simplified,

Eg From 90 -40 million years ago low grade diamond deposits were being deposited along this river.
From 40 - 10 million years ago higher grade material was being deposited along this river.

Therefore the shallower deposits are richer than the deeper.

Although being able to treat 1000m3 /day MUS have only recovered 24 cts.

But they have ramped their activities up very quickly and are having an honest go.
I wish them luck and who knows, but until they can confirm a bit better grades I will be staying on the sidelines.