You used a UnionPay credit card, right? In fact, the problem is very simple. When you swipe a UnionPay credit card, the exchange rate is calculated based on the exchange rate published by UnionPay on that day. UnionPay's exchange rate is only released once a day and does not change after it is released until the next day. You can call UnionPay to check, or go to Baidu "UnionPay Exchange Rate QuickEasy Finance" to check the UnionPay exchange rate of Hong Kong dollars online on QuickEasy Finance. You can click "Hong Kong Dollar" to enter the detailed mode, where you can check historical exchange rate information and just compare it. So if you make four transactions, the UnionPay exchange rate will be the same because they were made on the same day.

The exchange rate you are comparing may be the exchange rate published by a commercial bank, which changes in real time and is different from the UnionPay exchange rate. Sometimes the exchange rate is higher and sometimes the exchange rate is lower.

The exchange rate is set by UnionPay, so you may not get the correct answer when you call the bank. Because the bank staff are not familiar with UnionPay’s exchange rate mechanism.