Banter and General Comments, page-7735

That's not uniform either. As converters would be optimized to remove impurities from feed they get from their offtake partners.
And battery producers would optimize battery chemistry to match chemistry they get from converters.
Thus battery grade carbonate or hydroxide from 1 producer might not be of same value to 2 different conversion plants (as it costs them more to remove impurities they aren't optimized for). And further that product might not be of same value to battery producers as even the slightest difference messes with the battery chemistry (reducing lifespan, energy density, increasing failure probability, etc). Thus they might not value the chemistry of product sold by 2 different conversion plants using the same chemical made by 1 miner.

This is just generalized example of sort of issue with generalised spot pricing.
It makes a lot more sense for offtake partners and contract pricing between miners/conversion plants/battery producers than spot pricing. And I'm sure they talk to each other about their specific requirements.