I would imagine the current economic situation in Argentina (inflation of 25% per year, plus the USD currency restrictions) plays a part in the discrepancies -- though the "because they can charge more" argument has a lot of validity.

The official exchange rate closed at ARS 4.98 per USD, but the blue rate has been moving significantly higher, now at 7.9 per USD.

FYI, when booking a MEX-SCL-IPC trip from LAN this week, I tested booking the trip on three of their country sites (Mexico, US, and Chile), and all three returned the same fares. No difference.