In Instant Match events, there is high volatility. The demand and supply of shares fluctuate on a real-time basis. So, in all probability, by the time you see the price and execute the orders, the price has already moved up or down. Another reason may be the number of shares you are willing to buy/sell. If the number of shares is on the higher side, then you might end up paying a greater price than what you originally saw on the events page.