The quality of USPS "tracking" is dependent upon manual scanning of the barcode during every step along the way .. in many cases, the vendor has a sub-quality label printed where the barcode is not easily readable by the scanner and thus, in order for it to be in the system, the number must be input manually which most won't do because of efficiency requirements ..
As well, the original concept of what we now commonly refer to as tracking, was "Delivery Confirmation" .. it was never intended to be a real tracking system ..
The good news is, based on the sheer number of USPS deliveries in a year, they do an incredibly good job ..