Trades are executed at the live price on the underlying exchange at the time your order is received, as a result the price that you invest at may differ from the price displayed. Further to this you are buying at the Offer Price which is the price at which sellers are currently prepared to sell.
The difference between all three prices quoted is an indication of the current interest investors have in this share. The bigger the difference between the prices, the smaller the interest/liquidity in the share i.e. there are limited buyers and sellers of this share. Investors should be cautious when buying or selling shares with lower liquidity.