Delivery is based on routes, which are designed to take 8 hours to fulfill on average. If there's a heavy parcel day, bad weather, coverage for sick workers, etc, the 8 hours may be 11 hours.

Now, they stop working after 8 hours in most cases and leave whatever is left in a pile to be done later. Just like with computers, when you develop a backlog in a system optimized for throughput, things tend to get worse.