"In my experience, Amazon drivers will mark packages as delivered when they haven’t gotten around to delivering yet."
Packages delivered by USPS, the country's national mail carrier, are often marked as delivered 1-2 days before they are actually delivered.
The first couple of times this happened to me, I stepped outside after seeing the delivery notification from Amazon, and got worried that someone had taken the package. But now I know it's a common issue due to the way USPS collects and displays data: