Over time, you will notice that the number of emails delivered will become increasingly less than the number of recipients to which you are sending your message. This occurs because previously bounced email addresses are considered "undeliverable" by the server and subtracted from a mailing. For example, if you have a list of 1,000 recipients and 20 emails "permanently" bounced on the first mailing and then 10 emails bounced on the second mailing, on the second mailing the delivered statistic would be somewhere around 970.
NOTE: Do not try to do the math. The mail server reports delivery confirmations and bounces separately. It is possible for a message to produce no bounce or delivery confirmation. It is also possible for an email to “temporarily” bounce or be blocked by a spam filter. In this case, an email might bounce the first time, but be successfully delivered the second time.