Liquidity and order size
A small order may be filled close to the best available price. A larger order can consume several levels of the order book, producing a different average price.
ExchangeWales guide
What is crypto slippage?
Slippage is the difference between an expected market price and the average price at which an order can actually be executed.
Fast markets
Prices can also move while an order is being prepared or executed. More volatile markets can create a larger difference between a recently displayed price and actual execution.
Fixed service quotes
A fixed exchange quote can include a buffer for market movement and liquidity during the payment window so the receive amount remains defined for that request.