As you may know, sent and delivered aren’t the same. A sent email can get bounced, leading to the inequality in sent and delivered counts. So, as a general rule of thumb, Sent = Delivered + Bounced.
In most cases, if the mail doesn’t get delivered, it will be bounced. In extremely rare instances, the mail might be discarded. But this depends on the email service providers, i.e., Gmail, Hotmail, etc. In this case, there might be some discrepancies in the counts shown.
If a mail is landing in your recipient’s spam folder, it’s also considered as delivered.