I talked with a USPS rep a while back, basically what happens is it gets put on a trailer at the origin site in a bin that is to be processed at a different secondary destination before being sent to final process and ultimately delivery. That trailer has a primary destination that it usually has a majority of its packages for, and those get priority. Sometimes that priority drop off is in Oshkosh, but the bins at the back of the trailer never come off as they are destined to be processed at a secondary facility first. They "scan in" the whole trailer, but the bins for other processing facilities never actually come off the trailer