The visible price is not unlimited
The best bid or ask can represent only a limited quantity. Larger executions may use several order-book levels and therefore have a different average price.
ExchangeWales guide
Why crypto exchange rates differ from public exchanges
A public exchange displays trading-market prices from its order book. A request-based exchange service displays a final service quote for a defined payment window.
The market keeps moving
A public price can change while a customer is copying an address and sending a blockchain payment. A fixed quote needs to account for that time risk.
Service quote
The final ExchangeWales quote can reflect liquidity, market movement protection, network and processing costs and operational risk. The exact amount to receive is displayed before the request is confirmed.