MailPoet counts some subscribers as "subscribers" when they are not, therefore, they won't receive newsletters but still they count as "subscribers" for the plan's quota anyway.
This total that counts towards the active plan's quota is "subscribed", "unconfirmed" and "inactive" (when sending with MailPoet Sending Service). You only exclude "unsubscribed" and "bounced".
I request a change for this behaviour, only count as real subscribers (towards the quota) anyone with the status of "subscribed".