I can guess the reason, it usual be the same for many games.. Commercial/Dealing reason..
If your money rate is too large and the game port in to the difference area which has lower currency rate or customer habit (so they can be able to buy less).. it tend to multiply the rate. For example, 1usd=7770 'A' country coins. And 1usd for 1 game coins. How to trade game coins with 100 'A' coins so that game coins will be multiply there. The spending may be the same rate (multiply) but it makes customer feeling bad if they paid 7770 money for just 1 game coins more than for 7770 game coins and spend that amount in game.