I initially thought the same thing but looking at September and...

I initially thought the same thing but looking at September and December quarter production compared to shipments there appears to be a 3-4-3-4 shipment pattern forming. Gxy can not sustain 4 x 15000t shipments per quarter at current production rate around 209Kt/pa. You also have to take into account that not all production can be sold in a quarter due to timing of shipments.

September Q 2017
Concentrate produced - 47 075t
Concentrate sold - 41 834t

December Q 2017
Concentrate produced - 52 139t
Concentrate sold - 58 904t

March Q 2018
Concentrate produced - estimated 52 250t
Concentrate sold - estimated 45 000t