What is the difference in audience calculation between scheduled deliveries and immediate? Is there any difference in what time or how the audience is selected?
The email audience for scheduled emails will be calculated shortly after midnight on the day of delivery. When an email audience is calculated, that means the recipients are being chosen to receive the email. In other words, the system determines who should receive the email at midnight based on your delivery qualifications, such as Target Groups, Do Not Email Groups, Delivery Filters, etc. If group membership changes between midnight and delivery, then it will not have an affect on who will receive the email because the audience has already been chosen. If a user action occurred between midnight and delivery that should have added or removed them from your delivery list, it will not change who the intended recipients are for your scheduled email. The audience was already selected before the user action occurred.
In summary, immediate deliveries will calculate the audience immediately before delivery, whereas scheduled emails will calculate the audience after midnight on the day of delivery.