- The first returns customers that do not have any orders whose total price is less than 1000.
- The second returns customers that have any orders whose total price is greater than or equal to 1000.
In other words, imagine a customer who made one order of total price less than 1000, and another order of total price greater than 1000. Because of the smaller order, the customer record would not be included in the first result set, but because of the larger order, it would be included in the second result set.