The problem is your bank. HKD to USD should not be swinging between a fx rate of $7.9HKD/USD and $7.5HKD/USD when HKD and USD is pegged around $7.7HKD/USD. There would be some day-to-day
variations but the difference here is pretty wide.
Seems like your bank is charging you a fx transaction fee on both the credit
and debit transactions.