It's not so much that they can't predict the price it's that every dollar is worth significantly more on the black market as the official market rate. Compare the dollar blue (Blue) row with the offical bank rate (Oficial). 13.19 vs 8.57. Many people change every peso they earn to dollars immediately. (www.dolarblue.net)
I believe that the banks are mostly Dollar->peso conversions and are extremely supply limited or cutoff from going the other way, and the black market is a both ways market.
