As we all know, Inventory Management folks handle each flight very differently, depending on historical figures to predict future demand. Every day of the week also can be handled differently than another day. Given that information, it is easy to understand why some flights might be always overbooked, while others never are.

Actually, domestic and international have different guidelines at NW.

For domestic flights, the general rule is to never overbook F. And in fact, quite often F seats are held back from being sold if Y is historically heavily overbooked. As long as weather issues and mechanicals do not exceed what they have forecasted as "normal" then everything usually works out just fine. In abnormal situations, though, it is possible that things could turn out very bad when it comes time to close the door of the plane, but usually it is only the standbys who might get left behind and everyone with confirmed reservations will be accommodated.

For international flights, depending on the historical averages, sometimes WBC can be overbooked by up to 2 passengers. Gate agents and supervisors have told me they "hate it" when that happens, because they sometimes are forced to downgrade a paying customer to a Y seat and refund a part of their fare. No matter how they look at, this is a very difficult and awkward situation to handle. But they also know that the average no-show rate for international flights runs around 8% and that there is always the possibility of misconnects, so they just deal with it on a case by case basis as part of their job.

In the end, only two things really matter: as many seats as possible are filled with "paying" customers; and most importantly, the revenue from the flight is the highest amount they could possibly have received that day.