A simple answer to this question. Card numbers generated are subject to being purchased from Mastercard. When ever a Credit Card company, Bank or Travel Money company uses Mastercard / Visa for their Cashless options, they have to pay for those card numbers generated and to be maintained.
I don’t work for Revolut, but the company I work for are subject to this and their Travel Money Cards are subject to fee’s and retail exchange rates… so for a fiver I’d count myself lucky because you’ll be saving more with Revolut than other “Cash Passport” alternatives.