The price is negotiated every 6 months now at the request of one...

The price is negotiated every 6 months now at the request of one of the parties (I can't remember which side initiated it?) and not annually as was in place at the start. So the contract certainly isn't airtight.

I would certainly be expecting the buyers to be using every loophole possible to get as low a price as possible. The buyers aren't a charity and a contract is in place for mutual benefit. You'd be wrong to think GXY is controlling the puppet here and getting lofty prices above those disclosed by other producers of equivalent product.