If you're on a two-lane road, whether it's legal or not to make left turns, it's often easier to wait to deliver packages opposite the mail box until you're driving back down the street on the side the houses are. I used to deliver mail on a road that was like that--heavily traveled, dips and rises in the road making it difficult to see ahead for traffic, and 90 degree corners where you didn't know if someone was coming. Of course, why your carrier put a "sorry we missed you" slip in your box before actually making the delivery to you, I dunno.
As for your FedEx issue, there's a reasonable explanation, actually. FedEx drivers deliver to a greater area than postal carriers. In my town, there are 18 mail routes but FedEx uses only five trucks for that same area. Postal carriers deliver to approximately 500 homes per route. A FedEx driver delivers in an area with about 2000 homes and businesses. If a package slips behind a seat or drops down behind a bunch of other packages in the back of the truck, the driver will attempt to look for it but can only take so much time before moving on. By the time that package is found, they could be miles away.
--If it's a Ground/Home Delivery package, the truck could literally be all the way across town from you. If the driver is lazy, they'll just code it attempted or unable to complete delivery due to road hazard, weather, or some such. If the driver isn't lazy, they would have gone back to deliver it.
--If it's Express, there's a greater chance that the truck could return to your area. Express has blocks of time in which they have to deliver packages, so they criss-cross back and forth over the same area in order to deliver their packages in the time slot requested by the shipper or recipient. But time is tight, so they may code your package because it's simply not possible to reattempt delivery to you.
"I'm yet another resource-consuming kid in an overpopulated planet raised to an alarming extent by Hollywood and Madison Avenue, poised with my cynical and alienated peers to take over the world when you're old and weak!" - Calvin, "Calvin & Hobbes"