This is because of after hours trading. Even after the market closes there is still trading going on and that trading is what is causing the price of a stock to move up and down.
Today a lot of stocks did open up a lot higher then they closed at, but that it not always the case. Stocks can also open lower then when they started or at the same price.
I personally think that there was a lot of panic selling happening the last couple of days and “big money” was trying to get in on the dip which pushed stocks up higher.
