at the start and end of day there is a period of time called the open price and closing price auctions. during this time all offers and bids are put on hold and then at the end of the period a formula is used based on the overlapping bids and offers to come up with an 'opening' price and a 'closing' price. The purpose is to prevent deliberate manipulation of the opening and closing prices and to provide an orderly open and close to the market. You were probably looking at your stocks between 4 and 4:10 p.m.