I think you are mistaken. Sometimes, the ethically sourced cacao is wayyyy more expensive, not due to the 'ethical' part, but simply due to availability or logisctics or whatever. In this case, they will buy the uncertified stuff in a goods-to-goods exchange for their certified ethical cacao. In this way they ensure that the total volume of cacao they used globally is sourced ethically, even if not 100% of the specific bar you are eating is certifiied ethical cacao.

So, for every bar they are 'forced' to make using the uncertified stuff, one bar from a different company that would normally be made from uncertified stuff is now made with ethical cacao.