CiviCRM appears to default to adding imported contacts with is_deceased set to null when no value is set in the imported field. Such contacts are currently excluded from bulk e-mail lists.
It would probably be better to include such contacts in bulk e-mail lists, just as they are for postal mailing lists.
There's more discussion here: http://civicrm.stackexchange.com/questions/1968/when-sending-mailing-incorrect-amount-of-contacts-displayed
I'll see if I can submit a patch.