As you are making a market order, there is no guarantee that the market will be the same when your order makes it to the stock exchange.
If you send an order overnight for execution in the morning, the market price for a stock can differ from where it closed overnight. For market orders during the trading session, you are more likely to get the order executed at a price very close to the current trading price.
In both instances, you can either get price improvement (i.e. it's better for you) or it can be slightly worse.