By overbooking, for 156 seats on an aircraft, Easyjet can sell tickets to 165 people and gain the extra revenue while knowing it is extremely unlikely 156 people will turn ip at the gate. Yes, that's an extra 400 or 500 pounds per sector just by doing some statistical analysis of previous passenger records. 6 sectors per day across a large fleet comes to seriously big cash per year - well worth the cost of a few statisticians on the payroll and the fairly rare EU261 denied boarding compensation.