When an order is sent to the orderbook in spot market, the matching engine will match it with available orders on the opposite side (buy versus sell and vice versa). Matching may or may not happen immediately depending on your type of order.
In a volatile market, open price (the price at which you place your order) and average price (the price at which your order is matched) may not be the same price. This is called "slippage." It can also happen when the orderbook is not populated enough (thin orderbook).