Bitfloor's "average price" for buys
The average price is calculated by taking into account the current market conditions (i.e. other buyers/sellers) against the amount you wish to buy/sell. Given that the market is always changing, your actual average price may vary. When you finally place a market order, the system will execute against the best available prices at the time.

If you would like to have more control over the pricing and order execution, use limit orders. These may not execute immediately (depending on how you price them and their size) but you will be able to control those factors.