Your package gets scanned at the sending post office and put into one of these cages with a placard with that cage's destination or "zone" on it.
The cages are loaded onto trucks and taken to a distribution center and then broke down into other cages heading in the direction where your package is going.
The placards on these cages are scanned with your package in them, your package is not individually scanned because according to the placards they know where its going.
When you look up your tracking number it is tracking the cage that your package is in, not your specific package.
Your package could get tossed into the wrong cage and end up in the wrong post office yet the tracking number would show you where its supposed to be.
The next time YOUR package gets scanned is when it arrives at the final post office where it will either be picked up by you or delivered to your address.
Every package is not individually scanned in a distribution center or every time it arrives and departs. Just the cage gets scanned.
That's probably about as clear as mud.
All of this mail was going to four small rural town post offices. It was loaded onto the truck in three cages and had to get broken down so it could be delivered to the correct post office. The only package that actually got scanned individually was that blue and gold bag in the bottom left of the picture. That's because its express over night mail. It does get scanned at each distribution center.
You can see the placard that gets scanned on the cage on the left side, that white piece of paper that gets printed and taped onto each cage by the sending facility.
This was the week prior to Christmas and that's about 4 times the regular amount of mail. Looking at the picture I can't see any reason for any package to ever get mis-delivered.
If you were to visit and go inside a large USPS distribution center you would wonder how the hell they ever get anything delivered. Its quite the process and amazingly accurate for how many pieces of mail they handle ever hour, 24 hours a day 365 days a year.