Can someone help me understand why our price for our 2nd...

Can someone help me understand why our price for our 2nd shipment isn't higher? Because I would like to know what would be the reasonable price for the next shipment if the percentage change isn't applied in a mostly straightforward manner.

Based on my nickel prices that I searched on markets business insider website the price of nickel on Dec 24th was $20293 and the next date on Dec 29th was $20317. The announcement was released on Dec 30th saying the ship left Dec 26th with the price for the shipment at US $14.8m for 10865t. So we can assume we got a price from either Dec 24th or Dec 29th for this shipment. Based on what we sent and what we were paid, it was $1362.17t.

According to the announcement on 17th Feb the next shipment we received US $14.6m and sent 9409t so receiving $1551.71t. This was sent on 15th Feb, and the nickel price on 15th Feb was $23950 and on 16th Feb it was $23902.

The price we received went up 13.91%. But the price of nickel difference was at least 17.65% (that's if we assume we got the prices for 16th Feb and Dec 29th). If we assume we got the prices of Dec 24th and Feb 15th the difference is 18.02%.

Why didn't our price go up at least 17.65%? This would be $1602t and then we would have received $15.073m (US) or at 18.02% we would receive $15.126m. At the exchange rate used in the announcement of 0.7122 this would be an AUD price for the shipment of 21.16m or 21.24m. An extra 650-750k is a lot of money in one shipment.