They tend to package and label orders as they come in, that's the only thing you can do to be efficient -- you can't let them build up.
But then they only drop off (or get pickup) 2x/week, e.g. Tues and Fri. Which might be fine if that's what their shipping times indicate.
But then the buyer gets confused because they assume it was mailed immediately, which it wasn't. But there's no way for a seller to print shipping labels from eBay in advance without eBay marking it as "shipped".
It gets even more confusing because with bulk pickups or dropoffs, they often don't even get scanned when the carrier receives them. They won't show as actually in the carrier's hands until they reach the first major hub, which can easily be a day or even two later.
AIUI, Amazon's policy is that credit cards don't get charged until the order ships.
As such, some shadier folks will create the label long before the item is actually shipped. However, since the label has been created, the order is now marked as "shipped" and the credit card is charged even though nothing has been packed, let alone actually shipped.
I assume they do this to avoid complaints of slow delivery when the sender takes a whole to post the item.
I have multiple times seen an "Out for Delivery" package switch to "Delivered" or "Delivery Attempted" at 10pm, presumably when the driver ended their shift and didn't want to be penalized for the undelivered packages. They usually showed up the following day.
Exactly this, it's infuriating.
And you can usually tell because a) it's marked as delivered at a time rounded to a perfect hour, like 2:00 pm or 9:00 pm (not 8:34 pm), and b) there's no delivery photo, when there always is otherwise.
But yeah, it's the driver not being able to make all deliveries (probably not their fault), but needing to fake the metrics. Usually they drop it off the next day or two days later, but other times it just gets lost, and it's harder to get a refund from the seller because it says delivered. So e.g. eBay will side with the seller in a dispute.
I feel like this one happens because the driver needs to meet quota today, so they scan the package delivered today, then when they are in the area tomorrow they actually deliver the package.
Unfortunately, this makes "delivered today" not a reliable indicator of "I actually received the package today".
Amusingly, for some obscure software, I write those emails ;)