It could mean that the package has arrived at your post office but even though it says delivered it may have not actually been placed in your mailbox yet.
The package may have just gotten scanned and just hasn't been placed in your mailbox yet or the USPS worker just hasn't placed the yellow card or key into your mailbox to access the package.
We're all human and do get sidetracked with things so maybe a customer came in while the USPS worker was scanning your package and didn't get your package put into the mailbox right away.
I would wait and check later in the day and see if it's there or go in and ask the USPS worker if it's there.