Form the article:
“Large central mail processing facilities are located between the various post offices they serve to distribute mail efficiently. These processing facilities have mail-sorting equipment that can handle standard letter-size mail. Individual post offices report the order of their mail delivery routes to these central processing facilities. The individual addresses are keyed into the sorting equipment computer in the order in which they are delivered. Then each day, letter-size mail is sent through the sorting equipment to place it into delivery order for individual mail carriers. The presorted mail (DPS) is placed in mail trays and delivered every morning along with parcels and unsorted mail pieces to every post office. This presorted mail can be taken by mail carriers directly to the street to be delivered. Individual carriers may place this mail into their sorting case along with larger pieces to avoid working from several trays while driving.”
It still looks like carriers get presorted mail and then have to add the unsorted bulk mail.
If you read it again, even the presorted gets sorted one more time.
It is only presorted by route/delivery driver. The driver actually sorts it again.
I should add, also note that it is only for letter size items.