There are many reasons why this could be, and I can not tell without a list of caches and a deeper investigation. But if you have recent such finds, it is likely that Project-GC does not yet know that the caches are archived. If that is the case, it will correct itself.
Also, logging after it's archived isn't enough. It has to be on a date after it was archived, but I assume you understood that. Just mentioning it. This is not true, I was thinking about another page at Project-GC, not the stats-number mentioned above.
For more information see the FAQ under How up-to-date is the data.