Because question #3 on the Western Union FAQ was reiterated by Patrick2006 in the form "Why has the rate of Western Union been lower than the blue dollar by so much in the past month?", and one cannot attempt to answer that very, very frequently asked question without discussing the other rates, whether CCL or MEP or "dólar xxxxx" or whatever.
To be clear for those who are either new or humor-impaired, there does not actually exist a Western Union FAQ. The closest thing is a summary* I wrote back in May 2022, from which I here quote paragraph 4 -
WU sets its own exchange rate. This usually varies somewhere between the CCL rate and the Blue rate. There are many exchange rates here in AR, and explaining them all is not within the scope of this post. This WU rate can vary from hour to hour, or it can remain the same for days or even weeks, though the latter is uncommon. As with all things AR, there are no certainties, and Your Mileage May Vary. WU does not treat all customers the same. We don't know why or how they are divided, or into what categories; all this is opaque. Welcome to Argentina.
*pinned at the top of the Expat Life forum. Some info, (such as promo codes), is outdated, but the basics remain the same.