You may experience a delay in sending the email if you choose [Send Now] instead of pre-scheduling your email distribution. Send now places you in the queue for sending behind what emails are already in the queue. Pre-scheduling saves your spot in the queue ahead of Send Now. If there are a lot of emails in the queue already yours will go out in the order received. Since there are multiple sending mechanisms the systems go round-robin between those in the queue which is why you may see a lag in the total send.
This system is true of all hosted email marketing systems. It's typically rare but does periodically happen.