I've got that kind of mailbox, too. A few times, some of my mail has been inadvertently stuffed into other adjacent boxes. And I've sometimes gotten other peoples' mail as well. Maybe his mail got stuffed into an unused box (which would explain why it didn't get forwarded to him). And maybe the mail carrier paid extra close attention after their encounter and made sure his mail hit the correct box thereafter. Or maybe not... :-)
I have no idea. But it went on for months, so that would have been a very stuffed box.
Yeah, there was definitely something else going on...
I think even the junk mail would have been too much for the box to hold in all that time. Admittedly, he probably didn't get as much as I did. But, still...