Turnips and Carrots could be priced equally per tonne, and still be worth trading because although you might think all root vegetables are substitutable, it turns out you can't make carrot soup with Turnips.

It's always worth remembering trade involves use values as well. We don't only trade for asymmetric profit, and there are things like hedging which include a yield where both can acknowledge future risk, and price accordingly.

I'm probably ignorant of some magic economist reason why the words are fluid and don't mean what I think they mean: this always seems to be the case talking economics from the stuffed armchair.

Another take on this is that we can agree to facts and disagree to consequences. Same information, different conclusions.