An email address under the status unsubscribed is a person who chose to unsubscribe from this mailing, while an email address that fell under not sent and then under previously unsubscribed is a person who unsubscribed in a previous email mailing.
Why is that?
When a person unsubscribes, we show it under the status Opted out (unsubscribed) for that email mailing. But by previously unsubscribed, we mean that a person unsubscribed in a previous email mailing and can therefore no longer receive mailings.