The issue where a customer continues to receive emails despite unsubscribing and being deleted from the data extension likely arises because the email address in All Subscribers still exists and is active. In Marketing Cloud, the All Subscribers list acts as a master list overriding subscription statuses in individual data extensions. Thus, even if a customer is removed from a specific data extension, if their status in All Subscribers is not updated to 'Unsubscribed,' they will continue to receive communications.