I think delivery times depend on how far away you are from a hub.
For example, I live in a city where there are two post offices on either side of town. I live smack dab in the middle of town but slightly to one side so I got the PO for that side. Since I'm on the outside edge of their "hub" in town, I get mail towards the end of the day (usually between 4 and 5).
I imagine that if I lived closer to the PO that I'd get my mail sooner. Its really frustrating when I get a check or a bill or something and I want to mail it out the same day but I can because I just received it as everything is closing