The price you see after clicking 'Buy' or 'Sell' on the home screen - and entering an amount - is the average price of your buy/sell. Your buy/sell goes through our live order book, so if your order buys/sells all of the market price orders, it will start buying/selling the next best-priced orders to fill your trade. So the price you see after you enter the amount you're looking to buy is reflective of that, which is displaying the average price of all of those orders.
This is more common for larger-sized orders, and is called 'Price Slippage'. This is why we always have the exact amount of digital currency you will be getting under Transaction Summary, for you to review before you complete your purchase.
Additionally, If you have a specific price in mind you are looking to buy at, you can input a Limit order in our Pro Trade exchange for that exact price.