I am not sure I understand your question but I will guess as to what you are asking.
I think you are asking why the "To:" address doesn't show all of recipients. When you select more than one recipient, the list of email addresses who receive the email are added to the BCC email header.
Here is what a header looks like when I select four users:
From: "Development Admin" <firstname.lastname@example.org>
Reply-To: "Development Admin" <email@example.com>
Content-Type: text/html; charset="UTF-8"
To: "Development Admin" <firstname.lastname@example.org>
Bcc: email@example.com, firstname.lastname@example.org, email@example.com, firstname.lastname@example.org
There are options within the Plugin Settings to specify the From Sender name and/or email address which will affect the header (see email@example.com above).
If you are just looking at the header in your email client, you will never see the BCC content by design. To see the header like I've shown above, I enabled debug which causes it to go to the PHP error log.