Strange amount of emails sent vs. actual contacts in list

hello,
I recently just sent out my first email blast with send press, and while it works great, I noticed something strange. I only have 3,163 recipients in my list and the report states that it sent 3,879 emails, and when I was sending the email it stated that there were 4,163 recipients. Is this a bug in the plugin or just a simple error that I should not worry about?

Many thanks,
Jimbo

http://wordpress.org/plugins/sendpress/