One thing you have to understand is that outlook does not actually look at send/receive dates to determine what is archived. It looks at modifed dates. If an old email is opened and modified in some way, the new modified date is what will determine what is archived.
You can add the “modified” header to your inbox folder and sort by that header. I am willing to bet the emails left behind have a more recent modified date than a sent or received date.
I found this out when a manager came to me complaining his outlook would not archive anything. In researching I discovered when you open your email for the first time in cached mode, ALL of your email modified dates change to the day they were cached on your pc and therefore nothing will archive.