Currency exchange is an activity similar to many others: you have individuals who buy and sell currencies like the Guatemalan Quetzal in exchange for euros to/from a retailer (the one that sells/buys from us quetzales).
If they do not stock enough number of quetzales, they get them from a wholesaler company. The difference between what they have paid for the quetzales and what they charge you when you buy them is the profit margin they get.
Nevertheless, in this currency exchange market, it may be the case that the currency provider or bank will not acquire the notes from a wholesale company, but alternatively gets them from a consumer who comes to Spain with quetzales.
In this case, in addition to obtaining a profit for buying the Guatemalan Quetzal banknotes from this traveller, they impose on you another margin when they offer you the currency. Double win!