During message creating process, when you select more than one list, it will make a sum of all subscribers, and that number will be displayed while creating message. Then, when it starts importing users into temporary table for send, it will throw out duplicates, so the number might be smaller in the queue.
When the sending process starts, 1-2-All will gather all prepared subscribers, loop through all of them, personalize your message for everyone separately, and then it will call mail() function to send that email for every subscriber.
Since 1-2-All is a PHP script, it relies on PHP functions/features. PHP sends email using the following function: $mailSent = mail('to@domain.com', 'Subject', 'Content');
When mail() function is called, PHP on your server pushes your prepared email message to server's MTA (mail transfer agent) queue which is responsible for delivering that email. If $mailSent returns TRUE, that means that your MTA accepted it and 1-2-All will ONLY THEN increment successful send count.
As you can probably see, this is a demanding process for a web server. However, since 1-2-All is a PHP software, it has to depend on other things such as your server/PHP/MTA configuration, filters, etc...
The point is: PHP script can report successful sending ONLY if your MTA returned true. But, sometimes (when there is a high load), your MTA might reject that email, so it won't be even shown in MTA logs. But 1-2-All will still report SENT, since it got that response. 1-2-All cannot make any influence on that.
