The Euro to Hong Kong Dollar exchange rate is really easy to understand. Like all markets, there are wholesale companies and retail companies.
Banks and currency suppliers are retailers that buy from currency wholesale companies the banknotes they estimate to offer to travellers at the professional exchange rate (interbank market) and add their profit margin.
They then offer those banknotes to travellers at their selling price of the day, winning that differene.
For example, if you want to buy HK dollars, you contact your bank or currency supplier and pay the sale price of the day they display. Such rate already includes a profit for the currency supplier.