Email reminders are sent based on schedule, which is always related to the date & time when the unpaid order was created.
The reminders aren't sending emails to orders which met the condition in the past. E.g., You enable 1st reminder to send email five days after order creation, and at the same time, you have orders older than five days. These orders won't get this reminder as the specified time already passed.
Once the order is marked as paid or canceled, the sending of emails stops automatically.