Ann: Appdx 4D and H1 FY20 Financial Statements, page-5

Hello HTL,
As others have suggested the base metal concentrates are sold in irregular lumps, ie. by the ship load. AMI owns the product until it is loaded on to the ship. Usually 2 or 3 ship loads per quarter so there is always some product sitting in trains, at the port, and at the mines.
Regards silver there are two factors here.
1 The silver recovery is not particularly high (80% I guess).
2 The Cu, Pb, Zn smelters never pay for all the silver in the concentrate they get. This is just another way the smelter ensures they make money and cover the refining costs. So AMI only sees some of the silver in the base metal concentrate.
For an explanation of some of these look at the fine print attached to the Mineral Resource and Ore Reserve Statement June 2019 announced last June.