Maybe the issue is your location. Canada Post had a reputation for having a rule “the truck does not leave until it is full”.
There is logistics software which anticipates trucks that are not going to be full enough at each stage of a shipment, and holds back prior steps in the transit chain so that they do not have any underfilled trucks.
Such software existed in the 1970s, so it is quite conceivable that Amazon has recently discovered this can be done.
I spent a few years working for a company which provided software for Fortune 500 manufacturing and distribution companies.
I saw what looked like this behavior one of my Amazon purchases a couple of weeks back.