Say 1 XOF (Cefa) = 3 NGN. It'd seem like the Cefa is stronger than the naira. But if it takes 1 NGN to buy (say) 3 baskets of tomatoes 🍅, and it takes 4 XOF to buy the same quantity of tomatoes, then, in reality, the naira is faring better than the Cefa.

Exchange rate != PP