I imagine it is dilution. The mineralised zone is probably narrower than the minimum mining width so they have to mine surrounding rock. Therefore more tonnes at a lower grade.
AN1 CuFe sells its Crossroads gold royalty to Northern Star for $4M cash; says better than waiting for mine launch
Week 46 Wrap: Copper hits 2mth low as Trump threatens net zero pace; US rally fades; HotCopper’s picks