Bids and asks
Buyers place bids and sellers place asks. The best visible price usually represents only the nearest available orders, not an unlimited quantity at one price.
ExchangeWales guide
Crypto order book explained
An order book is the list of buy and sell orders currently available on a trading venue at different prices and quantities.
Market depth
If an order is larger than the quantity available at the best price, execution can continue through additional price levels. The resulting average execution price can therefore differ from the headline market price.
Why this matters
A request-based exchange service may use market data and liquidity sources to form a final service quote rather than simply copying one number from a public order book.