Bob_McBob is correct. I worked for UPS during undergrad in a quasi-night manager position.
The truck that picks up the package takes it back to a central hub where it gets routed and sent back out to its destination. It's possible that the intake hub was not the same one that delivers to you, so it can take extra time.
Also, the volume is huge. The place I worked at had something like 250,000 packages going through every day. The amount of planning involved was nothing short of amazing. And yes, the employees took things seriously. One of the "nightmare" scenarios was when a truck broke down at the end of the day. Sometimes NDA or 2DA packages had to be driven to the airport and put on commercial, non-UPS flights, with a bunch of planning to have someone get it on the other end.
So, yeah, the packages can take a day or two longer. But these days, I don't fault UPS, FedEx or the USPS for these things. 99% of the stuff I buy or send isn't time sensitive. As long as it shows up undamaged, I'm happy.