If the original travel cost is: RMB830 (equals to US$100), CHT will quote you US$100 according to the current exchange rate in China (suppose the rate is: US$1=RMB8.3).
CHT will ask their bank to bill RMB830 to your credit card. When your credit card receives the RMB830 charge, your creditcard will have to pay RMB830 at the exchange rate applicable at the time which may be lower or high than 8.3. If the exchange rate is lower than 8.3, you will have to pay more than US$100 for RMB830. If the exchange rate is higher than 8.3 you pay less than US$100 for RMB830. This is the reason that you may find the charges may differ slightly from what has been quoted by CHT.
Here is an example:
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