This is just my theory/understanding, but here goes.
WU and other remittance companies that are sending dollars to Argentina and paying out in pesos are the other half of the CCL. Consider, if they do the CCL thing then they are receiving ARS here in Argentina, and paying out dollars in the USA. Obviously they want to unload those pesos as quickly as possible, so the remittance business is the other half of that cycle; they receive our dollars in the USA and pay us with pesos here in-country. If the pesos start to build up, then they raise the "WU rate" to attract more remittance business. But when there is not as much money being sent out of the country via CCL, they can drop the rate and make more profit on the deal.
So, if my understanding is correct, then the decline of the WU Rate indicates that, for some reason, not as much money is being sent out of the country via CCL. But that seems counter-intuitive, no?