Because the government has about 6 different dollars they recognize (or don't). It's all made up fantasyland of denial. check out dolarhoy.com for all the different rates. It's easier to say you have less inflation when you have an artificial official rate and do not recognize your actual rate (blue) which is based in reality.
WU uses CCL for the rate at which they track. This is the rate Argentines are forced to use to sell stocks and whatnot in foreign countries. As you can see, this costs them a lot of money since the rate is so poor for buying dollars. The government is discouraging it's citizens from taking money out of the country. For folks of other citizenship with foreign accounts this is very good.