@771c7916 I guess overbooking is much more likely in places where there's a lot of connections per area. If you have to provide a lot of connections in a street full of appartments then either you need expensive routers that can provide multi-gigabits bandwidth on a single fiber, or lay a lot of fibers. Both are expensive options which makes connections expensive. overbooking seems a lot more efficient then, especially if usage is mixed commercial/residential. 

 @771c7916 The consumer contracts people get sold is usually for a maximum speed, not a guaranteed speed.  Professional contracts tend to state the maximum speed and the overbooking ratio-> how many customers share the same trunk. The lower the overbooking ratio, the more likely you are to reliably get the maximum advertised speed.