Ummm that is how. No trading during the weekend means Sunday's night open can be different from Friday's close. Also weekday globex opens can be different from the prior RTH close if there is some funnymental news like a market leading company's earnings announcement that comes out after hours. Or even just a plain old order imbalance.
Ever heard of books....Try Reminiscences Of A Stock Operator. That is if your interested in market history.
I'm afraid our range is going to tighten after earnings season is over. The first Quarter sure was a blessing.