Content and Item Details download count does not jive

When I go to Reports > Details, the download count for a particular content title is 7,636. When I drill into the content by clicking on the hyperlinked iTunesID, I'm presented with a list of ALL detail items under the content item. When I add up the number under the download column, my total is only 7,388. I expected those numbers to match. Is that expectation wrong? If so, could you please explain why?

Thanks!