i wonder

As I have been preparing for the long journey back to america I have encountered something which I find particularly odd. If I want to buy US dollars ($) here in china they cost me 6.84RMB . If I want to buy them in america they will cost me 6.2RMB now that doesnt sound like much. Let us say that you have $100. You could get 620RMB in america or 684 here in china. Where would you shop? For me it is exactly the opposite, instead of paying 684RMB(what i get paid and exist in) to get $100 in china,I can pay 620RMB and get the same there. Even dollars are on sale in america. What a country. Anyway I learned all this because there is a limit for how much RMB to $ that one person can exchange in a day. Everyday. Pointless.