For the report for a mailing from CiviMail, delviered and opened counts are always the same. Our client reports that for a number of mailings, the number opened always equals the number delivered. They don't think this is very realistic ...
I sent a test mailing to two different email addresses (both mine), viewed only one of them, but both showed as delivered and as opened.
CiviCRM 4.5.4 on Drupal.