Why would UPS send a package to the post office to deliver?

This week I have a package that's being sent to my house through UPS. When it was sent, the shipper said, "estimated arrival date Fri 08/12/2011". Thursday morning I went to the UPS site to see where the package was and at 6:30am it was at the local UPS. Seeing that it was already here in town got me thinking that it would be loaded onto a truck and delivered a day earlier than expected. I work nights, and it was close to the time I normally go to sleep so I thought I would go to sleep and the package would be waiting for me when I woke up.  Around 3:30pm I woke up, looked out the door and nothing was there. I didn't really think anything of it at the moment since UPS has been known to deliver to my neighborhood as late as 7pm. Checked their tracking site again and the most recent update said  "8:15 A.M. Destination Scan". If the destination is my house why is it not here. It had an entry for a 6:30am Arrival Scan and for 8:15am Destination Scan. Those seem like 2 names for similar things.     
Digging through the UPS website I find:
Arrival Scan: This scan is an electronic record indicating the shipment has been received at a UPS facility. Shipments may travel through several UPS facilities throughout their journey. The shipment is moving; however, there may be several days between scans if the shipment is going cross-country or moving between countries.  
Destination Scan: This scan is an electronic record indicating the shipment has arrived at the local UPS facility responsible for final delivery.
 
The day went on and around 9pm I'm sitting around waiting for more work to come in and I decide to check the UPS site again. The latest update on their site now shows 10:31 A.M. Package transferred to post office and 1:02 P.M. Received by the local post office. "Why would the post office have my package from UPS??"
Doing more digging on the site I see:
Transferred to Local Post Office for Delivery: As requested by the sender, this shipment has been transferred to the local post office for delivery to the final destination. Allow one to two additional days for delivery. 
 
Why would a company request that the post office deliver a package rather than UPS who were the ones who they trusted the package with up to this point? Why would they want to involve more people and increase the chances of something going wrong? Why wouldn't the sender and UPS want the package arriving a day earlier than expected? It sure would make them both look better to be able to serve customers like that.
 
They had already stated that the estimated arrival date was Friday so it isn't late yet. I'm not angry at anyone involved, just confused.
 
EDIT/UPDATE:
I forgot to say why I dislike receiving packages through the US Post Office. Our mailbox is not in front of our house, it is on the next block over from us. When packages are delivered to us through the Post Office one of two things could happen. 1. Our normal delivery driver drives her car to our house, honks the horn and we run out to pick up the package from her. If we are not home at the time, she leaves a note in the mailbox and we have to pick up the package at the post office the next day. OR 2. We get the guy who is a temp/fill-in driver who puts the package into a plastic shopping bag, ties it to the mailbox and leaves it hanging there at the side of the street waiting for anyone to come by and take it. 
 
Friday came and so did the package. Just as I had feared, it did not get brought to the house, it was left at the mailbox. Although, in a surprise twist it was not in a bag tied to the mailbox. The package was left sitting on top of the mailbox. At least if it were tied to the mailbox then it would take some effort for someone to take it. There it was, a $150 package left out in the open for anyone to take. Luckily nobody did.