I was told years ago that 'officially' ez did not overbook. How could a computer put more people on a flight than seats? Someone suggested they might on some routes and not others. I suppose a computer could do that. It might be common for business men commuters to make multiple early bookings on Mondays & Fridays at the lowest pricers and then throw away the unused.However, the excellent policy of allowing you onto an earlier flight, space available, might negate that idea. Sadly, this allowance is only on the return sector of the same booking reference. Why do they have such a good customer convenience idea and then make it restrictive? People often book a single out & then back. This does not allow you to catch an earlier flight.
Overbooking on the business weekend flights is highly lucrative as the few over bookings will be at the highest prices. Statistics will show how real the gamble is and it could be a nice little earner.