The number of unique subscribers per month is almost always more than what the mailing is. Why is this so?
The reason for this is that in the upper right corner you will see the total number of members in the account. The tags you send to don't include all members, but only those that are active in those specific tags.
The options we offer today are either the number of emails per month or the number of members of your account. See below how it is announced in Rule:
If you want to review your basic settings for an upgrade, please contact us through firstname.lastname@example.org and we'll administer it for you.