i did a few months as a postie ('till i could get a job in my more usual line of work) and i have to say that there does seem to be that a system where a regular postie would be held to account for such missing items is being undermined by the apparent policy of moving people around the walks a lot, where the postman/woman is less part of the community as an occasional visitor.
never witnessed any outright dishonesty, other than some posties taking a chance and scanning a special delivery as delivered before 1 o'clock, knowing that they'd miss the deadline otherwise and that the item could be shoved through the letterbox. very naughty, and a sackable offence, but no harm done i suppose...