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Hey Cecile, it depends. Sometimes you will have emails that are listed as delivered although they've been caught by a spam system that's in place. But, the email server won't always send out an response that the email was trapped so it looks like it was delivered. If it returns a code saying that it was not delivered for some reason then it's listed as a bounce and won't be included in your delivered totals.
Hope this helps.
To complement Dory's points, see this discussion: How accurate is the timestamp on Marketo's activity log? in the thread, Justin Cooperman provides some details on how and when the "delivered" info is being received by Marketo, or not...