Even if you use the LWT, the mail will still have to be put in pouch bag...
I know some that have very easy rounds in my office and some on singleton rounds that use trays/boxes but they are a tiny minority; and some simply use the van like a bicycle/shuttle wagon to go from point ot point - thereby using small bundles.
Anyway, when you have an undelivered small letter special, they will need to be secure until you get back to the van; and most people leave them in the pockets of the pouches because you do not want them falling out when you lift bundles or packets/parcels out of the pouch.
It is easy to forget the letter is in the small pocket of the pouch - genuine mistake - but often it is more to do with not being methodical, rushing or being distracted etc.
Easily done especially with PDAs. The paper system at least give you a chance to tally the delivered and undelivered and therefore such mistakes were limited and could even be spotted straight-away by office posties.