Board Game Arena

I am new to the game so am not sure if this is a bug. If I have a settlement next to the ship offering 2:1 on wheat the "rates"below the building cost shows wheat at a rate of 2:1 but how do I get that rate? If I try to do it as a maritime trade I gat charged 4:1

It means you can use 2 wheats to exchange for any resource, not the reverse.